4300 lines
84 KiB
Text
4300 lines
84 KiB
Text
0:00:09.469,0:00:11.309
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Hello my name is Marshall Kirk McKusick
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0:00:11.309,0:00:15.389
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and I've been around as long as dinosaurs
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and mainframes have ruled the world
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0:00:15.389,0:00:18.429
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which is to say the sixties and seventies
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0:00:18.429,0:00:22.460
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however by 1970s a new breed of mammals had begun to show up
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on the scene
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0:00:22.460,0:00:24.240
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known as mini computers
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0:00:24.240,0:00:28.230
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although they were just toys in the 1970s they would soon grow
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0:00:28.230,0:00:31.689
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and take over most of the computing market
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0:00:31.689,0:00:33.150
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In 1970
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0:00:33.150,0:00:37.910
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at AT&T Bell laboratories two researchers Ken
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Thompson and Dennis Ritchie began developing the
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0:00:37.910,0:00:39.900
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UNIX operating system
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0:00:39.900,0:00:42.040
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Ken Thompson who had been an alumnus at Berkeley
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0:00:42.040,0:00:46.100
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came back on a sabbatical in 1975 bringing UNIX
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with him
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0:00:46.100,0:00:47.539
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In the year that he was there
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0:00:47.539,0:00:51.330
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he managed to get a number of graduate students interested
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in UNIX
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0:00:51.330,0:00:53.940
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and by the time he left in 1976
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0:00:53.940,0:00:56.829
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Bill Joy has taken over in running the UNIX system
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0:00:56.829,0:01:00.470
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and in fact continuing to develop software for it.
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0:01:00.470,0:01:04.339
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Bill began packaging up the software that had
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been developed under Berkeley UNIX and
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0:01:04.339,0:01:05.779
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and distributing it
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0:01:05.779,0:01:08.040
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as the Berkeley Software Distributions
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0:01:08.040,0:01:12.310
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whose name was quickly shortened to simply BSD
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0:01:12.310,0:01:16.330
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BSD continued to be distributed with
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yearly distributions for almost fifteen
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0:01:16.330,0:01:17.490
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years
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0:01:17.490,0:01:21.920
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initially under Bill Joy and later under others including
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yours truly.
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0:01:21.920,0:01:24.860
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By the late 1980s interest had began to grow
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0:01:24.860,0:01:27.400
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in freely redistributable software
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0:01:27.400,0:01:30.170
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so a number of us at Berkeley began separating
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out
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0:01:30.170,0:01:32.649
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the AT&T proprietary bits of BSD
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0:01:32.649,0:01:35.710
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from those parts that were freely redistributable.
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0:01:35.710,0:01:40.590
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By the time of the final distribution at BSD
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in 1992
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0:01:40.590,0:01:43.620
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the entire distribution was freely redistributable.
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0:01:43.620,0:01:45.909
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I live in a capsule history here
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0:01:45.909,0:01:48.009
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but if you're interested in the entire story
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0:01:48.009,0:01:50.789
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I have this three-and-an-half hour epic
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0:01:50.789,0:01:54.590
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which is available from my website www.mckusick.com
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0:01:54.590,0:01:58.200
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that gives the entire history of Berkeley.
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0:01:58.200,0:02:00.239
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Following the final distribution from Berkeley
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0:02:00.239,0:02:01.450
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two groups sprung up
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to continue supporting BSD
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0:02:03.600,0:02:08.080
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the first of this was the NetBSD whose primary
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goal was to support
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0:02:08.080,0:02:10.459
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as many different architectures as possible
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0:02:10.459,0:02:14.769
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everything from your microwave oven all way
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upto your cray XMP
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0:02:14.769,0:02:19.409
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In fact today NetBSD supports nearly
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sixty architectures.
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0:02:19.409,0:02:22.419
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The other group that sprang up was FreeBSD.
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0:02:22.419,0:02:28.239
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Their goal was to bring up BSD and support
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as wide a set of devices as possible on the
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0:02:28.239,0:02:29.719
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PC architecture.
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0:02:29.719,0:02:36.549
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They also had a goal of trying to make the
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system as easy to install as possible to
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0:02:36.549,0:02:39.309
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attract by a wide group of developers
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0:02:39.309,0:02:42.319
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I chose to work primarily with the FreeBSD
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group
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0:02:42.319,0:02:43.740
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both doing software
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0:02:43.740,0:02:46.140
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and also together with George Neville Neil
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0:02:46.140,0:02:51.069
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writing this book ""The Design and Implementation
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of the FreeBSD Operating System"".
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0:02:51.069,0:02:52.060
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Together with this book
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0:02:52.060,0:02:53.959
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I developed a course
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0:02:53.959,0:02:56.500
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which runs for twelve chapters
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0:02:56.500,0:02:58.179
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and thirty hours.
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0:02:58.179,0:02:59.749
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The purpose of this video
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0:02:59.749,0:03:01.089
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is to give you a taste
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0:03:01.089,0:03:02.819
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of that course.
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0:03:02.819,0:03:07.249
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What follows are excerpts from the first lecture
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of the course
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0:03:07.249,0:03:11.139
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which of course you can also get from my website
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www.mckusick.com.
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0:03:11.139,0:03:13.069
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0:03:13.069,0:03:17.739
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Enjoy.
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0:03:17.739,0:03:22.239
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This class is nominally about FreeBSD
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because well
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0:03:22.239,0:03:26.379
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that's what I know best and that's what
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the textbook is organized around
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0:03:26.379,0:03:29.979
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but the fact of the matter is that it's really
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0:03:29.979,0:03:32.339
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a class about your UNIX and that
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0:03:32.339,0:03:36.539
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really covers sort of the broad range of things
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in the open source arena as its FreeBSD
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0:03:36.539,0:03:37.689
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in Linux
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0:03:37.689,0:03:38.899
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which of course
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0:03:38.899,0:03:41.159
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you use a lot out
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0:03:41.159,0:03:41.550
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and
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0:03:41.550,0:03:44.349
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it also covers a commercial systems
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0:03:44.349,0:03:46.950
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%uh Solaris, HP-UX,
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0:03:46.950,0:03:49.279
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AIX and so on.
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0:03:49.279,0:03:52.419
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I am going to tend more towards the open
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side
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0:03:52.419,0:03:56.389
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open source side of things.So it's really
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going to be more FreeBSD in Linux than it's
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0:03:56.389,0:03:57.579
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going to be
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0:03:57.579,0:04:00.849
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Solaris and HP-UX and so on.
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0:04:00.849,0:04:06.959
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For the most part at the level of this course
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we're dealing with the interfaces to the system
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0:04:06.959,0:04:07.329
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and
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0:04:07.329,0:04:11.599
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the fact that the matter is a those interfaces are highly
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standardized at this point
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0:04:11.599,0:04:12.060
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and
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0:04:12.060,0:04:15.280
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whether it's FreeBSD or Linux or Solaris
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or whatever
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0:04:15.280,0:04:19.460
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the Socket system call has to do the same
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thing, it has to have the same arguments
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0:04:19.460,0:04:20.150
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in that,
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0:04:20.150,0:04:23.909
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it has to have the same effect
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0:04:23.909,0:04:27.319
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and so until you get down to the really nitty
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details
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0:04:27.319,0:04:29.600
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of how they actually go about implementing
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that
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0:04:29.600,0:04:31.960
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the differences are relatively minor.
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0:04:31.960,0:04:35.830
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So I would say that sixty to seventy percent
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of the material that I'm covering
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0:04:35.830,0:04:40.779
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is just as true for FreeBSD as it would
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be for Linux
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0:04:40.779,0:04:42.580
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or for Solaris
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0:04:42.580,0:04:44.659
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%uh AIX is a little bit
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0:04:44.659,0:04:45.629
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sort of off in the weeds
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0:04:45.629,0:04:48.709
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%uh as is HP-UX
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0:04:48.709,0:04:51.099
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but luckily we don't have to worry too much about
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that.
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0:04:51.099,0:04:54.569
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Okay so
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0:04:54.569,0:04:59.279
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the other thing is that I'm going to assume that
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all of you have used the system. I get
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0:04:59.279,0:05:00.910
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really sort of worried when people
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0:05:00.910,0:05:04.249
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you know raise the hands and ""Hey, what's a Shell?""
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0:05:04.249,0:05:07.990
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or I don't
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put a lot of code up but a one piece of code and someone said ""Why
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0:05:07.990,0:05:11.819
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are there two pipe symbols in the middle of
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that that If statement?"".
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0:05:11.819,0:05:15.740
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No we're not programming the Shell we're programming
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in C.
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0:05:15.740,0:05:19.970
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So hopefully you can tell the difference between
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Shell scripts and C code.
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0:05:19.970,0:05:21.990
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so okay but I am but am gonna assume
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you haven't really looked inside the system.
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0:05:24.610,0:05:28.289
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So I gonna start everything to at a very
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high level.
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0:05:28.289,0:05:32.969
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The problem is I have already discovered you come
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from a lot of different sort of
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0:05:32.969,0:05:33.819
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backgrounds
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0:05:33.819,0:05:35.180
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and
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0:05:35.180,0:05:36.280
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levels of knowledge
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0:05:36.280,0:05:37.900
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and so
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0:05:37.900,0:05:42.620
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the way that I find works best to sort of
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be useful to everybody is that three pass
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0:05:42.620,0:05:43.860
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algorithm
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0:05:43.860,0:05:49.060
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so what I will do is start the first pass a very
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broad brush high level
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0:05:49.060,0:05:50.569
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description of what's going on
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0:05:50.569,0:05:54.719
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and then I will go back and I'll go through the
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same material again but at a lower level of
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0:05:54.719,0:05:55.300
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detail
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0:05:55.300,0:05:59.939
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then I finally go back and go through a very nittily
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low-level of detail
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0:05:59.939,0:06:04.649
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and the fact of this is if you are learning new stuff
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as I'm doing the high-level thing
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0:06:04.649,0:06:08.649
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you are gonna be utterly washed by the time I get to
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low level niggly details
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0:06:08.649,0:06:10.699
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but since I'm going to do it topic by topic
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0:06:10.699,0:06:14.190
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when I get to the end of one of those nearly
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low level niggly details
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0:06:14.190,0:06:17.900
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I'll give you a clue as I will say ""Brain
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reset, I'm starting a new topic"" so even if
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0:06:17.900,0:06:19.330
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you're completely lost
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0:06:19.330,0:06:23.530
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you can now start listening again plus I'm gonna get
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the broad brush up again.
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0:06:23.530,0:06:27.059
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okay and for those of you that know a lot of
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this stuff already
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0:06:27.059,0:06:31.770
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you'll probably find the broad brush rather boring
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0:06:31.770,0:06:35.759
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but by the time we get down to nearly low level
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details I think you'll actually
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0:06:35.759,0:06:37.860
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pick up some things that you will find
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0:06:37.860,0:06:39.710
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useful and interesting.
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0:06:39.710,0:06:43.759
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So in this way hopefully everybody will
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get some
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0:06:43.759,0:06:47.699
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useful percentage of material out of the course.
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0:06:47.699,0:06:49.599
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I am gonna start out by just
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0:06:49.599,0:06:53.089
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walking through and giving you the
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0:06:53.089,0:06:56.919
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outline of what we're going to try and do here
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here
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0:06:56.919,0:07:01.169
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As I said we're going to go roughly
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0:07:01.169,0:07:03.270
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just about two-and-an-half hours of lecture
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0:07:03.270,0:07:04.729
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about two hours forty minutes
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0:07:04.729,0:07:06.499
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per week
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0:07:06.499,0:07:07.619
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and
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0:07:07.619,0:07:11.770
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so we will start off this week with an introduction.
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0:07:11.770,0:07:13.860
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This is as I said we're going to start from the
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top
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0:07:13.860,0:07:15.749
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and then just start working our way down
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0:07:15.749,0:07:19.350
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so the general thing I'm going to do is
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to talk about the interface
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0:07:19.350,0:07:21.439
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%uh which is something that you
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0:07:21.439,0:07:25.319
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are presumably fairly familiar with since
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you've worked with that system
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0:07:25.319,0:07:27.249
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and then
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0:07:27.249,0:07:29.739
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you have to sort of layout terminology
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0:07:29.739,0:07:32.080
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although we use normal English words
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0:07:32.080,0:07:34.419
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they have
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0:07:34.419,0:07:38.580
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sometimes rather bizarre meanings compared to their
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common usage
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0:07:38.580,0:07:39.220
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and
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0:07:39.220,0:07:42.330
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so I will just sort of lay out the terminology
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lay out the
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0:07:42.330,0:07:45.750
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the way we talk about how the system is structured
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0:07:45.750,0:07:50.780
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and this week we will also talk about the
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basic services ""What is it that the kernel is
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0:07:50.780,0:07:52.929
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providing for us?""
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0:07:52.929,0:07:54.060
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and then of course
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0:07:54.060,0:07:58.499
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we'll proceed to dive down in and and see how
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that is done
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0:07:58.499,0:07:59.970
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so here in
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0:07:59.970,0:08:01.400
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Week number 2
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0:08:01.400,0:08:05.450
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we're gonna look at the system from the
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perspective of
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0:08:05.450,0:08:07.039
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something that
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|
|
0:08:07.039,0:08:08.720
|
|
manages processes.
|
|
|
|
0:08:08.720,0:08:12.170
|
|
One way of looking at the kernel is it's really
|
|
just a
|
|
|
|
0:08:12.170,0:08:16.440
|
|
the resource manager and the resource that
|
|
its managing are things going to do with processes
|
|
|
|
0:08:16.440,0:08:19.460
|
|
So we'll look at a process, what the structure of
|
|
it is
|
|
|
|
0:08:19.460,0:08:20.649
|
|
and
|
|
|
|
0:08:20.649,0:08:23.559
|
|
talk about the different ways that they can
|
|
be structured.
|
|
|
|
0:08:23.559,0:08:28.379
|
|
Process can for example be an address space
|
|
and can have one thread running in it can have
|
|
|
|
0:08:28.379,0:08:29.749
|
|
multiple threads running in it.
|
|
|
|
0:08:29.749,0:08:34.620
|
|
so we'll talk about the different ways
|
|
that we think a process is.
|
|
|
|
0:08:34.620,0:08:38.480
|
|
We will look at the management of those processes
|
|
|
|
|
|
0:08:38.480,0:08:39.239
|
|
we've got
|
|
|
|
0:08:39.239,0:08:42.020
|
|
to lay out the bits and pieces that
|
|
need to be managed
|
|
|
|
0:08:42.020,0:08:44.660
|
|
and then talk about
|
|
|
|
0:08:44.660,0:08:47.190
|
|
how we do that.
|
|
|
|
0:08:47.190,0:08:51.740
|
|
we'll talk about jails.. this is something
|
|
that you currently find only in FreeBSD
|
|
|
|
0:08:51.740,0:08:55.060
|
|
hasn't made it into
|
|
|
|
0:08:55.060,0:08:56.320
|
|
Linux yet although
|
|
|
|
0:08:56.320,0:09:01.630
|
|
the concept is being actively worked
|
|
on so my guess is that you'll see that
|
|
|
|
0:09:01.630,0:09:03.500
|
|
fairly soon.
|
|
|
|
0:09:03.500,0:09:06.360
|
|
we'll also then talk about scheduling
|
|
|
|
0:09:06.360,0:09:10.579
|
|
which is in essence how we decide what gets
|
|
to run, when it gets to run, how long it gets
|
|
|
|
0:09:10.579,0:09:13.500
|
|
to run, etc.
|
|
|
|
0:09:13.500,0:09:14.330
|
|
okay
|
|
|
|
0:09:14.330,0:09:19.020
|
|
The week after that we will go into virtual
|
|
memory.
|
|
|
|
0:09:19.020,0:09:23.800
|
|
Signals aren't really part of virtual memory
|
|
but they didn't fit into next week's
|
|
|
|
0:09:23.800,0:09:26.400
|
|
material so I just would dropped that at the
|
|
beginning
|
|
|
|
0:09:26.400,0:09:29.850
|
|
but the bulk of Week 3 is going to
|
|
be
|
|
|
|
0:09:29.850,0:09:32.019
|
|
the management of Virtual Memory. So we've got
|
|
|
|
0:09:32.019,0:09:35.119
|
|
a bunch of physical memory, a bunch of
|
|
processes that are
|
|
|
|
0:09:35.119,0:09:37.940
|
|
trying to use their address spaces
|
|
|
|
0:09:37.940,0:09:39.590
|
|
and we will talk about
|
|
|
|
0:09:39.590,0:09:41.410
|
|
essentially how you will make that all work
|
|
|
|
0:09:41.410,0:09:43.510
|
|
It's called a virtual memory because it's
|
|
|
|
0:09:43.510,0:09:47.420
|
|
sort of a cheat. We promise you the world and
|
|
then we deliver you
|
|
|
|
0:09:47.420,0:09:51.480
|
|
as small number of pages as we think we
|
|
can get away with.
|
|
|
|
0:09:51.480,0:09:56.420
|
|
Okay. So the first three weeks then essentially
|
|
get us through
|
|
|
|
0:09:56.420,0:09:58.340
|
|
looking at the world as if it was all
|
|
|
|
0:09:58.340,0:10:00.560
|
|
all about processes.
|
|
|
|
0:10:00.560,0:10:03.880
|
|
Then in Week 4 we change gears. we say
|
|
okay well you know
|
|
|
|
0:10:03.880,0:10:07.570
|
|
the kernel isn't just all about processes. You can sort of
|
|
look at it orthogonally and you can
|
|
|
|
0:10:07.570,0:10:10.000
|
|
say it's really just a giant I/O switch
|
|
|
|
0:10:10.000,0:10:12.910
|
|
it's just like a traffic cop that's just managing
|
|
these
|
|
|
|
0:10:12.910,0:10:14.860
|
|
I/O streams
|
|
|
|
0:10:14.860,0:10:15.450
|
|
and
|
|
|
|
0:10:15.450,0:10:18.610
|
|
so let's look at it from that perspective.
|
|
|
|
0:10:18.610,0:10:19.310
|
|
And
|
|
|
|
0:10:19.310,0:10:24.740
|
|
we'll start with special files, again this
|
|
sort of the interface when you talk about UNIX
|
|
|
|
0:10:24.740,0:10:25.880
|
|
systems, when you talk about
|
|
|
|
0:10:25.880,0:10:27.950
|
|
what's normally /dev
|
|
|
|
0:10:27.950,0:10:34.170
|
|
interface that gets you access
|
|
to the various I/O streams that are available
|
|
|
|
0:10:34.170,0:10:37.220
|
|
and we'll look at how that's organized and
|
|
the structure of it
|
|
|
|
0:10:37.220,0:10:41.840
|
|
which used to be fairly simple but in the
|
|
last decade has gotten
|
|
|
|
0:10:41.840,0:10:43.670
|
|
incredibly complicated.
|
|
|
|
0:10:43.670,0:10:48.540
|
|
We will also talk about pseudo terminals in
|
|
job control
|
|
|
|
0:10:48.540,0:10:53.330
|
|
this is about as interesting as watching the
|
|
grass grow but unfortunately it's
|
|
|
|
0:10:53.330,0:10:55.490
|
|
a major component of the system
|
|
|
|
0:10:55.490,0:10:59.520
|
|
and especially people that deal with system
|
|
administration have to know far more about
|
|
|
|
0:10:59.520,0:11:06.520
|
|
this than they probably ever thought they
|
|
wanted to.
|
|
|
|
0:11:06.900,0:11:11.430
|
|
Okay we will then continue in Week 5 with
|
|
the kernel I/O structure,
|
|
|
|
0:11:11.430,0:11:16.090
|
|
We will start with multiplexing of I/O. The
|
|
kernel of course has done this
|
|
|
|
0:11:16.090,0:11:17.360
|
|
always
|
|
|
|
0:11:17.360,0:11:22.110
|
|
but we're really talking more about how do
|
|
we export I/O multiplexing to
|
|
|
|
0:11:22.110,0:11:25.970
|
|
user applications.
|
|
|
|
0:11:25.970,0:11:29.250
|
|
We will then move into auto configuration strategy
|
|
|
|
0:11:29.250,0:11:31.370
|
|
Auto configuration
|
|
|
|
0:11:31.370,0:11:32.770
|
|
is what happens
|
|
|
|
0:11:32.770,0:11:36.619
|
|
typically or historically I guess you
|
|
could say as the system boots.
|
|
|
|
0:11:36.619,0:11:39.500
|
|
so all that stuff that comes out about
|
|
|
|
0:11:39.500,0:11:40.810
|
|
what
|
|
|
|
0:11:40.810,0:11:43.550
|
|
hardwares are on the machine and how it's all
|
|
interconnected
|
|
|
|
0:11:43.550,0:11:47.350
|
|
all of that is tied up in auto configuration
|
|
|
|
0:11:47.350,0:11:50.040
|
|
and that used to happen just once it boots
|
|
|
|
0:11:50.040,0:11:52.000
|
|
but in modern systems today
|
|
|
|
0:11:52.000,0:11:55.839
|
|
it's an ongoing process. It happens at boot
|
|
but it also happens
|
|
|
|
0:11:55.839,0:12:00.550
|
|
anytime you plug a new I/O device, a
|
|
PCMCIA card,
|
|
|
|
0:12:00.550,0:12:03.680
|
|
or you remove a disk or you put in a new disk.
|
|
|
|
0:12:03.680,0:12:07.010
|
|
or any sort of activity that changes the I/O
|
|
|
|
0:12:07.010,0:12:08.360
|
|
structure of the machine
|
|
|
|
0:12:08.360,0:12:10.870
|
|
auto configuration has to get fired back up
|
|
|
|
0:12:10.870,0:12:13.050
|
|
and figure out what's disappeared
|
|
|
|
0:12:13.050,0:12:18.330
|
|
and cleanup and figure out what new has arrived
|
|
to configure it in.
|
|
|
|
0:12:18.330,0:12:19.320
|
|
and then we'll talk
|
|
|
|
0:12:19.320,0:12:23.870
|
|
a little bit about the configuration of the
|
|
device driver
|
|
|
|
0:12:23.870,0:12:27.390
|
|
this actually gets into an area that
|
|
|
|
0:12:27.390,0:12:28.660
|
|
is
|
|
|
|
0:12:28.660,0:12:33.440
|
|
one well let me just give it as a bit
|
|
of advice to the class especially those of
|
|
|
|
0:12:33.440,0:12:36.780
|
|
you who work in system administration.
|
|
|
|
0:12:36.780,0:12:42.010
|
|
You really want to be careful that
|
|
you don't learn too much about device drivers
|
|
|
|
0:12:42.010,0:12:44.670
|
|
because there is really these three things that
|
|
|
|
0:12:44.670,0:12:48.580
|
|
it's not good to learn about and if you do
|
|
learn about it it's really good to keep it
|
|
|
|
0:12:48.580,0:12:49.740
|
|
to yourself
|
|
|
|
0:12:49.740,0:12:51.949
|
|
because if you become an expert or
|
|
|
|
0:12:51.949,0:12:54.960
|
|
viewed as an expert in any of these areas
|
|
|
|
0:12:54.960,0:12:59.370
|
|
you will become the designated stuccy for
|
|
that and your site you'll never get to do
|
|
|
|
0:12:59.370,0:13:01.760
|
|
anything
|
|
|
|
0:13:01.760,0:13:02.610
|
|
but that
|
|
|
|
0:13:02.610,0:13:07.360
|
|
so The three things that I highly
|
|
recommend not learning very much about are
|
|
|
|
0:13:07.360,0:13:09.060
|
|
device drivers,
|
|
|
|
0:13:09.060,0:13:12.320
|
|
send mail configuration files
|
|
|
|
0:13:12.320,0:13:13.970
|
|
or anything having to do
|
|
|
|
0:13:13.970,0:13:19.350
|
|
with LDAP or anything in
|
|
that general domain
|
|
|
|
0:13:19.350,0:13:22.660
|
|
because as I say
|
|
|
|
0:13:22.660,0:13:24.900
|
|
that will become your life's work
|
|
|
|
0:13:24.900,0:13:25.920
|
|
and
|
|
|
|
0:13:25.920,0:13:32.920
|
|
there's other things that you might find more interesting.
|
|
""Do you have a question?""
|
|
|
|
0:13:33.870,0:13:36.659
|
|
so one of my students empathizes with my point
|
|
|
|
0:13:36.659,0:13:39.640
|
|
I believe you said you worked on that mail
|
|
system
|
|
|
|
0:13:39.640,0:13:43.120
|
|
so you you might know something about
|
|
Sendmail configuration files but you don't
|
|
|
|
0:13:43.120,0:13:47.850
|
|
have to answer that
|
|
|
|
0:13:47.850,0:13:52.100
|
|
okay so we're going to talk about what a device
|
|
driver does and really just sort of the entry
|
|
|
|
0:13:52.100,0:13:53.170
|
|
points to it
|
|
|
|
0:13:53.170,0:13:57.180
|
|
but we're not going to talk about how you
|
|
write such a thing, how you debug such a thing
|
|
|
|
0:13:57.180,0:14:01.490
|
|
or much of anything about it. I actually used
|
|
to teach an entire class believe it or not
|
|
|
|
0:14:01.490,0:14:02.720
|
|
about device drivers
|
|
|
|
0:14:02.720,0:14:05.849
|
|
but then I realized the error of my ways and I have
|
|
since
|
|
|
|
0:14:05.849,0:14:12.580
|
|
gone through and made a point of forgetting
|
|
every slide in that talk.
|
|
|
|
0:14:12.580,0:14:16.860
|
|
okay so then we will move on to File system
|
|
|
|
0:14:16.860,0:14:21.540
|
|
and as always we'll start at the high level
|
|
talk about the interface what is it that is
|
|
|
|
0:14:21.540,0:14:23.020
|
|
exported out of the system
|
|
|
|
0:14:23.020,0:14:27.840
|
|
and then we will start diving down in the C and
|
|
how do we go about implementing that
|
|
|
|
0:14:27.840,0:14:29.010
|
|
so
|
|
|
|
0:14:29.010,0:14:31.010
|
|
we'll start with the
|
|
|
|
0:14:31.010,0:14:32.560
|
|
so called
|
|
|
|
0:14:32.560,0:14:33.680
|
|
Block I/O system
|
|
|
|
0:14:33.680,0:14:36.140
|
|
it's historically been called buffer
|
|
cache
|
|
|
|
0:14:36.140,0:14:38.590
|
|
and you still hear it called that periodically
|
|
|
|
0:14:38.590,0:14:42.720
|
|
and the fact of the matter is that there isn't really
|
|
about buffer cache anymore, there is just one big
|
|
|
|
0:14:42.720,0:14:44.620
|
|
cache in it.Its the VM cache
|
|
|
|
0:14:44.620,0:14:47.810
|
|
and the Filesystem has a view into it
|
|
and
|
|
|
|
0:14:47.810,0:14:50.829
|
|
the processes have a view into it but at
|
|
the end of the day
|
|
|
|
0:14:50.829,0:14:54.660
|
|
you really don't want the same information
|
|
on two different
|
|
|
|
0:14:54.660,0:14:56.030
|
|
pages of memory
|
|
|
|
0:14:56.030,0:14:59.390
|
|
because that just leads to trouble.
|
|
|
|
0:14:59.390,0:15:03.390
|
|
But Filesystems think they have buffers and so
|
|
there's this maneuver where we make
|
|
|
|
0:15:03.390,0:15:06.149
|
|
these things that look like what historically
|
|
were buffers
|
|
|
|
0:15:06.149,0:15:08.830
|
|
that really just map into VM system
|
|
|
|
0:15:08.830,0:15:11.720
|
|
but they're still managed in the way that
|
|
they have been
|
|
|
|
0:15:11.720,0:15:15.020
|
|
managed historically
|
|
|
|
0:15:15.020,0:15:20.670
|
|
okay We will then get down into Filesystem implementation
|
|
the local file system if you will
|
|
|
|
0:15:20.670,0:15:23.400
|
|
and into also
|
|
|
|
0:15:23.400,0:15:25.730
|
|
soft updates and snapshots.
|
|
|
|
0:15:25.730,0:15:26.440
|
|
this
|
|
|
|
0:15:26.440,0:15:31.100
|
|
for the time being is something that you see
|
|
only in FreeBSD
|
|
|
|
0:15:31.100,0:15:35.310
|
|
the alternative to soft updates is journalling
|
|
which is %uh more commonly used
|
|
|
|
0:15:35.310,0:15:39.630
|
|
for example what is used by ext3
|
|
|
|
0:15:39.630,0:15:41.179
|
|
and so I'll go through soft updates and
|
|
|
|
0:15:41.179,0:15:45.260
|
|
a lot of the issues in soft updates are the
|
|
same issues that you have to deal with journalling
|
|
|
|
0:15:45.260,0:15:48.370
|
|
what is it that we're protecting and how do we
|
|
go about doing that
|
|
|
|
0:15:48.370,0:15:51.150
|
|
and the difference is in the detail.
|
|
|
|
0:15:51.150,0:15:54.630
|
|
There is actually a paper in the back to your
|
|
notes if this is something that interests
|
|
|
|
0:15:54.630,0:15:55.240
|
|
you
|
|
|
|
0:15:55.240,0:15:59.930
|
|
it's a comparison of journalling versus
|
|
soft updates that was done
|
|
|
|
0:15:59.930,0:16:02.120
|
|
about five or eight years ago.
|
|
|
|
0:16:02.120,0:16:08.460
|
|
and not to spoil the punch line but the answers
|
|
they both work about are the same
|
|
|
|
0:16:08.460,0:16:12.500
|
|
Okay snapshots again is something that
|
|
if
|
|
|
|
0:16:12.500,0:16:15.920
|
|
you've worked with things like the network
|
|
appliance box you're probably quite
|
|
|
|
0:16:15.920,0:16:19.640
|
|
aware of what snapshots are and how they do
|
|
or don't work for you
|
|
|
|
0:16:19.640,0:16:21.959
|
|
this is the same functionality
|
|
|
|
0:16:21.959,0:16:27.380
|
|
in the Filesystem implemented in a
|
|
somewhat different way
|
|
|
|
0:16:27.380,0:16:28.449
|
|
okay so this
|
|
|
|
0:16:28.449,0:16:31.940
|
|
Week 6 is really going to be the local
|
|
file system
|
|
|
|
0:16:31.940,0:16:34.750
|
|
the disk connected to the machine
|
|
that we are dealing with.
|
|
|
|
0:16:34.750,0:16:39.140
|
|
Week 7 then we get into multiple
|
|
Filesystem support so how do we abstract out that
|
|
|
|
0:16:39.140,0:16:41.190
|
|
Filesystem layer
|
|
|
|
0:16:41.190,0:16:46.430
|
|
and support Multiple Filesystems at the
|
|
same time so for example in FreeBSD
|
|
|
|
0:16:46.430,0:16:50.199
|
|
you can of course run with their traditional
|
|
fast Filesystem
|
|
|
|
0:16:50.199,0:16:54.540
|
|
but if you happen to like the Linux Filesystem
|
|
better or you have to share a disk
|
|
|
|
0:16:54.540,0:16:55.690
|
|
with a Linux machine
|
|
|
|
0:16:55.690,0:16:58.310
|
|
you can run the ext2 or ext3
|
|
|
|
0:16:58.310,0:17:01.020
|
|
and it will perfectly happily do that
|
|
|
|
0:17:01.020,0:17:01.620
|
|
so
|
|
|
|
0:17:01.620,0:17:05.589
|
|
we will have to look then at how do we provide
|
|
interface so that we can plug in all these different
|
|
|
|
0:17:05.589,0:17:09.260
|
|
Filesystems that we want to support
|
|
|
|
0:17:09.260,0:17:12.250
|
|
another area of which there's been a great
|
|
|
|
0:17:12.250,0:17:15.309
|
|
deal of growth at least in code complexity
|
|
|
|
|
|
0:17:15.309,0:17:17.840
|
|
is so-called Volume Management
|
|
|
|
0:17:17.840,0:17:19.370
|
|
so in the
|
|
|
|
0:17:19.370,0:17:24.480
|
|
good old days a Filesystem lived on a disk or
|
|
piece of disk and that was that
|
|
|
|
0:17:24.480,0:17:26.130
|
|
but in this day and age
|
|
|
|
0:17:26.130,0:17:31.150
|
|
that won't do any more so we aggregate disks
|
|
together by striping them or RAID
|
|
|
|
0:17:31.150,0:17:31.980
|
|
arraying them
|
|
|
|
0:17:31.980,0:17:33.380
|
|
or various other things
|
|
|
|
0:17:33.380,0:17:39.210
|
|
and we need a whole layer in the system just to
|
|
manage those disks
|
|
|
|
0:17:39.210,0:17:44.280
|
|
we'll then get to the as an example of an alternative
|
|
Filesystem we're going to talk about the
|
|
|
|
0:17:44.280,0:17:46.530
|
|
Network Filesystem or NFS
|
|
|
|
0:17:46.530,0:17:48.500
|
|
but that's not because this is
|
|
|
|
0:17:48.500,0:17:51.090
|
|
the world's best remote file system
|
|
|
|
0:17:51.090,0:17:55.240
|
|
or the cleanest design or any of the
|
|
properties you might hope that
|
|
|
|
0:17:55.240,0:17:57.049
|
|
such a class as this one would have
|
|
|
|
0:17:57.049,0:17:58.600
|
|
but it's ubiquitous
|
|
|
|
0:17:58.600,0:18:00.210
|
|
very widely used
|
|
|
|
0:18:00.210,0:18:01.350
|
|
and
|
|
|
|
0:18:01.350,0:18:06.850
|
|
so we're going to talk about that one
|
|
|
|
0:18:06.850,0:18:07.740
|
|
okay we'll
|
|
|
|
0:18:07.740,0:18:10.970
|
|
then once again switch gears in Week 8
|
|
|
|
0:18:10.970,0:18:17.120
|
|
and turn our attention to of Networking and
|
|
Interprocess communication
|
|
|
|
0:18:17.120,0:18:18.200
|
|
and
|
|
|
|
0:18:18.200,0:18:23.210
|
|
again we'll start from the very top so we'll
|
|
go through, we'll go with concepts, the terminology
|
|
|
|
0:18:23.210,0:18:24.450
|
|
that gets used
|
|
|
|
0:18:24.450,0:18:30.230
|
|
and what's the difference between domain
|
|
based addressing and an address domain you know
|
|
|
|
0:18:30.230,0:18:30.910
|
|
we'll go through
|
|
|
|
0:18:30.910,0:18:34.910
|
|
what the basic IPC services are,
|
|
|
|
0:18:34.910,0:18:39.080
|
|
essentially what are all the system calls that
|
|
have anything to do with networking
|
|
|
|
0:18:39.080,0:18:40.590
|
|
and
|
|
|
|
0:18:40.590,0:18:43.720
|
|
just sort of describe what each of them are
|
|
and I'm going to go through
|
|
|
|
0:18:43.720,0:18:45.830
|
|
a somewhat contrived example
|
|
|
|
0:18:45.830,0:18:49.840
|
|
that makes use of every one of those interfaces
|
|
|
|
0:18:49.840,0:18:52.860
|
|
and just to sort of show how they all connect
|
|
together
|
|
|
|
0:18:52.860,0:18:54.169
|
|
and for those of you that work
|
|
|
|
0:18:54.169,0:18:57.400
|
|
in networking or had done any kind of network
|
|
programming
|
|
|
|
0:18:57.400,0:19:00.480
|
|
if you're looking for a week to miss and the
|
|
Week 8 is the one to miss that's 'cause that's
|
|
|
|
0:19:00.480,0:19:02.780
|
|
the sort of most basic
|
|
|
|
0:19:02.780,0:19:04.210
|
|
lecture that I'm going to give
|
|
|
|
0:19:04.210,0:19:07.910
|
|
If you are not sure whether or not you need to
|
|
go through that, there is
|
|
|
|
0:19:07.910,0:19:09.540
|
|
one of the papers in the back
|
|
|
|
0:19:09.540,0:19:12.620
|
|
it is an introduction to Interprocess communication
|
|
|
|
0:19:12.620,0:19:18.279
|
|
read that paper if you say yeah yeah yeah
|
|
yeah yeah you are done with Week 8.
|
|
|
|
0:19:18.279,0:19:20.590
|
|
on the other hand if you don't come to Week
|
|
8
|
|
|
|
0:19:20.590,0:19:22.790
|
|
and then in Week 9 I say
|
|
|
|
0:19:22.790,0:19:26.860
|
|
I call on you and say alright what is it
|
|
|
|
0:19:26.860,0:19:30.560
|
|
that listen system call does and you
|
|
can't tell me
|
|
|
|
0:19:30.560,0:19:32.610
|
|
you're gonna get a demerit
|
|
|
|
0:19:32.610,0:19:34.340
|
|
okay
|
|
|
|
0:19:34.340,0:19:37.770
|
|
then in Week 9 we will get into the actual
|
|
|
|
0:19:37.770,0:19:41.419
|
|
networking implementation itself, we go
|
|
through system layers as we did
|
|
|
|
0:19:41.419,0:19:43.310
|
|
in all the other areas
|
|
|
|
0:19:43.310,0:19:44.130
|
|
and
|
|
|
|
0:19:44.130,0:19:48.330
|
|
we will spend a significant portion of that
|
|
class talking about routing
|
|
|
|
0:19:48.330,0:19:50.230
|
|
routing
|
|
|
|
0:19:50.230,0:19:53.610
|
|
for those of you that haven't had the pleasure
|
|
of dealing with it
|
|
|
|
0:19:53.610,0:19:55.540
|
|
is a black art
|
|
|
|
0:19:55.540,0:19:58.050
|
|
or at least a dark science
|
|
|
|
0:19:58.050,0:19:59.170
|
|
and
|
|
|
|
0:19:59.170,0:19:59.930
|
|
so
|
|
|
|
0:19:59.930,0:20:02.490
|
|
we'll talk about it
|
|
|
|
0:20:02.490,0:20:06.270
|
|
from the perspective first of all of what
|
|
do we do locally within the machine
|
|
|
|
0:20:06.270,0:20:10.090
|
|
and then what are some of the bigger strategies
|
|
that we can use for doing routing
|
|
|
|
0:20:10.090,0:20:11.910
|
|
enterprise
|
|
|
|
0:20:11.910,0:20:14.840
|
|
wide routing or
|
|
|
|
0:20:14.840,0:20:20.190
|
|
area wide routing something like throughout the
|
|
state of California or throughout the US whatever
|
|
|
|
0:20:20.190,0:20:25.379
|
|
this again like device drivers is really
|
|
just sort of a nickel
|
|
|
|
0:20:25.379,0:20:26.480
|
|
tour through the
|
|
|
|
0:20:27.800,0:20:31.820
|
|
what the choices are what that the basic
|
|
strategies are that are used
|
|
|
|
0:20:31.820,0:20:33.989
|
|
If you're thinking you're going to walk out
|
|
of here
|
|
|
|
0:20:33.989,0:20:36.110
|
|
knowing how to set up a routing well sorry
|
|
|
|
0:20:36.110,0:20:38.430
|
|
we are not going to get that far
|
|
|
|
0:20:38.430,0:20:41.559
|
|
but you should at least have a pretty good idea
|
|
of what the issues are
|
|
|
|
0:20:41.559,0:20:44.430
|
|
and what the general solutions are
|
|
|
|
0:20:44.430,0:20:48.950
|
|
okay then finally in Week 10 well not finally
|
|
but next few weeks and
|
|
|
|
0:20:48.950,0:20:52.380
|
|
we will go through the Internet Protocols
|
|
|
|
0:20:52.380,0:20:54.320
|
|
primarily TCP/IP
|
|
|
|
0:20:54.320,0:20:56.560
|
|
and this is
|
|
|
|
0:20:56.560,0:20:58.809
|
|
what are the algorithms that are used
|
|
|
|
0:20:58.809,0:21:01.030
|
|
and I'm putting a particular emphasis
|
|
|
|
0:21:01.030,0:21:03.050
|
|
for this particular class
|
|
|
|
0:21:03.050,0:21:05.080
|
|
on
|
|
|
|
0:21:05.080,0:21:07.730
|
|
changes that have been made in the protocols
|
|
|
|
0:21:07.730,0:21:14.310
|
|
to deal with a lot of the sort of attacks that
|
|
we've been seeing the SYN attacks and
|
|
|
|
0:21:14.310,0:21:16.880
|
|
that sort of thing
|
|
|
|
0:21:16.880,0:21:19.440
|
|
rather than just a straight
|
|
|
|
0:21:19.440,0:21:22.440
|
|
iteration of what the actual protocols
|
|
are
|
|
|
|
0:21:22.440,0:21:24.940
|
|
I'll talk primarily about IPv4
|
|
|
|
0:21:24.940,0:21:31.940
|
|
but I will also try and talk a bit about
|
|
IPv6 as well
|
|
|
|
0:21:33.510,0:21:35.850
|
|
all right so the first ten weeks are
|
|
|
|
0:21:35.850,0:21:38.100
|
|
sort of the kernel course
|
|
|
|
0:21:38.100,0:21:40.800
|
|
now we attack two weeks at the end
|
|
|
|
0:21:40.800,0:21:42.010
|
|
to talk about
|
|
|
|
0:21:42.010,0:21:43.990
|
|
sort of the bigger picture of
|
|
|
|
0:21:43.990,0:21:48.240
|
|
System Tuning,Crash dump analysis that level of
|
|
thing
|
|
|
|
0:21:48.240,0:21:52.940
|
|
The idea is to really consolidate what
|
|
we figured out or talked about in the first
|
|
|
|
0:21:52.940,0:21:54.710
|
|
ten weeks and
|
|
|
|
0:21:54.710,0:21:58.760
|
|
how that applies to tools that we have available
|
|
to us to
|
|
|
|
0:21:58.760,0:22:00.760
|
|
look at what the system is doing,
|
|
|
|
0:22:00.760,0:22:02.649
|
|
analyze what the system is doing
|
|
|
|
0:22:02.649,0:22:03.650
|
|
and hopefully
|
|
|
|
0:22:03.650,0:22:04.720
|
|
improve
|
|
|
|
0:22:04.720,0:22:07.130
|
|
the performance of what the system is doing
|
|
|
|
0:22:07.130,0:22:07.750
|
|
and
|
|
|
|
0:22:07.750,0:22:12.169
|
|
for the most part the kind of tuning that I'm
|
|
talking about is not
|
|
|
|
0:22:12.169,0:22:14.740
|
|
going in and hack hack hacking your kernel
|
|
|
|
0:22:14.740,0:22:16.510
|
|
because the fact that the matter is
|
|
|
|
0:22:16.510,0:22:18.600
|
|
most of the time you can't do that anyway
|
|
|
|
0:22:18.600,0:22:22.340
|
|
so it's more looking at it from the perspective
|
|
of saying
|
|
|
|
0:22:22.340,0:22:26.390
|
|
is this system running badly because it doesn't
|
|
have enough memory on it?
|
|
|
|
0:22:26.390,0:22:29.470
|
|
or is it running badly because there isn't enough
|
|
I/O capacity?
|
|
|
|
0:22:29.470,0:22:33.549
|
|
or is it running badly because it's got
|
|
enough I/O capacity but
|
|
|
|
0:22:33.549,0:22:35.940
|
|
certain drives are being overloaded
|
|
|
|
0:22:35.940,0:22:37.309
|
|
or is it
|
|
|
|
0:22:37.309,0:22:42.220
|
|
being overrun because we're simply trying
|
|
to do too much on this machine? etc.
|
|
|
|
0:22:42.220,0:22:45.440
|
|
so that's the sort of level of thing that we're
|
|
looking at it
|
|
|
|
0:22:45.440,0:22:47.080
|
|
but tied into
|
|
|
|
0:22:47.080,0:22:52.130
|
|
lot of concepts that we talked before so we can talk
|
|
about active virtual memory
|
|
|
|
0:22:52.130,0:22:53.710
|
|
and what that means
|
|
|
|
0:22:53.710,0:22:55.120
|
|
and
|
|
|
|
0:22:55.120,0:22:58.750
|
|
essentially measure what it is and hopefully
|
|
then you will understand in the context of what
|
|
|
|
0:22:58.750,0:23:00.690
|
|
we talked about in the VM section
|
|
|
|
0:23:00.690,0:23:03.990
|
|
what that really means
|
|
|
|
0:23:03.990,0:23:07.460
|
|
the Crash dump analysis is one of these
|
|
topics that
|
|
|
|
0:23:07.460,0:23:08.730
|
|
you are gonna love or hate
|
|
|
|
0:23:08.730,0:23:12.530
|
|
you actually have to deal with crashed
|
|
dumps
|
|
|
|
0:23:12.530,0:23:13.679
|
|
its people find it invaluable
|
|
|
|
0:23:13.679,0:23:15.580
|
|
and if you don't have to deal with Crash dumps
|
|
|
|
0:23:15.580,0:23:18.790
|
|
it's an incredible mass of boring detail
|
|
|
|
0:23:18.790,0:23:23.240
|
|
the only good part of it is that that's the
|
|
whole session is only about an hour long
|
|
|
|
0:23:23.240,0:23:25.529
|
|
If it interests you, listen closely
|
|
|
|
0:23:25.529,0:23:28.950
|
|
and if it bores you, well, its only an hour long
|
|
|
|
0:23:28.950,0:23:32.880
|
|
okay lastly we'll talk a little bit about
|
|
security issues
|
|
|
|
0:23:32.880,0:23:36.250
|
|
again this is really more to the tools that
|
|
are available
|
|
|
|
0:23:36.250,0:23:40.750
|
|
to deal with security staff as opposed to a
|
|
complete tutorial on
|
|
|
|
0:23:40.750,0:23:45.120
|
|
how to implement security so those of you
|
|
that deal with security
|
|
|
|
0:23:45.120,0:23:48.400
|
|
this is just gonna to be sort of security one oh
|
|
one
|
|
|
|
0:23:48.400,0:23:50.029
|
|
for those of you
|
|
|
|
0:23:50.029,0:23:51.500
|
|
that have but
|
|
|
|
0:23:51.500,0:23:54.399
|
|
you'll have to deal with it but haven't really
|
|
thought about it
|
|
|
|
0:23:54.399,0:23:58.549
|
|
it'll probably scare you to death and
|
|
you wonder how to keep the machines from
|
|
|
|
0:23:58.549,0:24:02.840
|
|
being hijacked everyday
|
|
|
|
0:24:02.840,0:24:08.030
|
|
Okay so that's in essence what we're going
|
|
to try and do here
|
|
|
|
0:24:08.030,0:24:15.030
|
|
anybody have any comments, questions, thoughts.
|
|
No? All right well.
|
|
|
|
0:24:16.130,0:24:17.840
|
|
Let's get started
|
|
|
|
0:24:17.840,0:24:22.180
|
|
we will be begin on page fifteen with an
|
|
overview of the kernel.
|
|
|
|
0:24:22.180,0:24:26.040
|
|
Hopefully nobody's lost yet.
|
|
|
|
0:24:26.040,0:24:29.310
|
|
What's a kernel? All right.
|
|
|
|
0:24:29.310,0:24:31.370
|
|
so starting at the very top
|
|
|
|
0:24:31.370,0:24:33.070
|
|
the big broad brush
|
|
|
|
0:24:33.070,0:24:35.140
|
|
what we have is
|
|
|
|
0:24:35.140,0:24:38.330
|
|
a UNIX virtual machine and
|
|
|
|
0:24:38.330,0:24:41.660
|
|
virtual machines are actually something
|
|
that has been around
|
|
|
|
0:24:41.660,0:24:44.539
|
|
as a concept since the sixties
|
|
|
|
0:24:44.539,0:24:48.919
|
|
difference is really just sort of the level
|
|
of the interface that people have dealt with
|
|
|
|
0:24:48.919,0:24:51.360
|
|
when they talk about Virtual Machines
|
|
|
|
0:24:51.360,0:24:53.610
|
|
in the 1960s
|
|
|
|
0:24:53.610,0:24:56.770
|
|
computers were these enormous things you would
|
|
have
|
|
|
|
0:24:56.770,0:24:58.870
|
|
your computer room would be something that'd be
|
|
|
|
0:24:58.870,0:25:01.909
|
|
three times the size of this conference
|
|
room if you had
|
|
|
|
0:25:01.909,0:25:03.230
|
|
a computer
|
|
|
|
0:25:03.230,0:25:05.530
|
|
the computer itself was
|
|
|
|
0:25:05.530,0:25:07.840
|
|
tall as a refrigerator freezer
|
|
|
|
0:25:07.840,0:25:08.950
|
|
imagine
|
|
|
|
0:25:08.950,0:25:13.909
|
|
five or eight or ten of these units
|
|
side by side that itself made up the computer
|
|
|
|
0:25:13.909,0:25:16.080
|
|
that would be one big
|
|
|
|
0:25:16.080,0:25:20.030
|
|
for the core processor and the one which
|
|
should be the floating point unit and several
|
|
|
|
0:25:20.030,0:25:24.080
|
|
of them that would be the memory the core memory
|
|
literally the core memory
|
|
|
|
0:25:24.080,0:25:29.110
|
|
and then they'd be other rows of these
|
|
disk drives which were about the size of the washing
|
|
|
|
0:25:29.110,0:25:29.660
|
|
machine
|
|
|
|
0:25:29.660,0:25:34.169
|
|
and then behind that since you couldn't store
|
|
everything on disks so
|
|
|
|
0:25:34.169,0:25:36.300
|
|
then you had rows of tape drives
|
|
|
|
0:25:36.300,0:25:37.880
|
|
and then you had this little
|
|
|
|
0:25:37.880,0:25:39.610
|
|
set of sort of
|
|
|
|
0:25:39.610,0:25:43.330
|
|
munchkins that would run around and and tend
|
|
to the machine and they'd mount tapes and take
|
|
|
|
0:25:43.330,0:25:46.710
|
|
off tapes and mount disc packs and remove disc packs
|
|
because
|
|
|
|
0:25:46.710,0:25:49.760
|
|
the drives themselves were very expensive and
|
|
so
|
|
|
|
0:25:49.760,0:25:53.110
|
|
you wouldn't just as today we have a
|
|
|
|
|
|
0:25:53.110,0:25:56.090
|
|
one spindle that was dedicated just to one set
|
|
of platters
|
|
|
|
0:25:56.090,0:25:57.130
|
|
you could take out a
|
|
|
|
0:25:57.130,0:25:59.460
|
|
set of platters and put in another
|
|
|
|
0:25:59.460,0:26:02.540
|
|
hundred megabytes set of platters and these are
|
|
platters that are
|
|
|
|
0:26:02.540,0:26:05.280
|
|
this big around and it's like six or eight
|
|
of them and
|
|
|
|
0:26:05.280,0:26:09.140
|
|
giant head assemblies they comes rumbling in and
|
|
out
|
|
|
|
0:26:09.140,0:26:12.440
|
|
anyway one of these giant giant machines
|
|
|
|
0:26:12.440,0:26:17.380
|
|
that costs many millions of dollars would run
|
|
at about ten
|
|
|
|
0:26:17.380,0:26:21.120
|
|
million instructions per second, 10 mips
|
|
|
|
0:26:21.120,0:26:21.630
|
|
and 10 mips
|
|
|
|
0:26:21.630,0:26:28.330
|
|
was more computing power than anybody
|
|
could possibly imagine using in a single application
|
|
|
|
0:26:28.330,0:26:28.880
|
|
just
|
|
|
|
0:26:28.880,0:26:31.050
|
|
by contrast you know this
|
|
|
|
0:26:31.050,0:26:34.070
|
|
four-year-old laptop here is probably on
|
|
the order of
|
|
|
|
0:26:34.070,0:26:36.440
|
|
one or two hundred mips
|
|
|
|
0:26:36.440,0:26:37.140
|
|
but anyway
|
|
|
|
0:26:37.140,0:26:40.760
|
|
people couldn't really view what we would
|
|
do with a lot of computing power
|
|
|
|
0:26:40.760,0:26:44.640
|
|
and the other thing was that you didn't have
|
|
a notion of sort of an operating system that had
|
|
|
|
0:26:44.640,0:26:45.890
|
|
applications running on it
|
|
|
|
0:26:45.890,0:26:46.760
|
|
because
|
|
|
|
0:26:46.760,0:26:50.160
|
|
everybody wanted to write straight to
|
|
the raw hardware
|
|
|
|
0:26:50.160,0:26:51.750
|
|
and so
|
|
|
|
0:26:51.750,0:26:55.900
|
|
what IBM who was a big manufacturer
|
|
of machines in those days
|
|
|
|
0:26:55.900,0:26:59.060
|
|
did what they came up with this thing called
|
|
the VM
|
|
|
|
0:26:59.060,0:27:00.770
|
|
and this was a little
|
|
|
|
0:27:00.770,0:27:02.549
|
|
you'd call an operating system really
|
|
|
|
0:27:02.549,0:27:05.130
|
|
but what it did is it cloned
|
|
|
|
0:27:05.130,0:27:09.270
|
|
independent copies of the machine that worked just
|
|
like the original machines so you could boot
|
|
|
|
0:27:09.270,0:27:11.769
|
|
something that you thought it was an operating
|
|
system
|
|
|
|
0:27:11.769,0:27:13.380
|
|
on top of VM
|
|
|
|
0:27:13.380,0:27:16.750
|
|
so you take one least ten mip machines and
|
|
it would clone
|
|
|
|
0:27:16.750,0:27:20.050
|
|
six identical one mip copies
|
|
|
|
0:27:20.050,0:27:22.030
|
|
and then you could boot
|
|
|
|
0:27:22.030,0:27:24.700
|
|
whatever you wanted on each one of those machines
|
|
so
|
|
|
|
0:27:24.700,0:27:29.510
|
|
if you were doing database stuff you would boot your
|
|
database because database cannot ran on the raw hardware
|
|
|
|
0:27:29.510,0:27:32.920
|
|
or if you're doing payroll who would boot up the payroll
|
|
program
|
|
|
|
0:27:32.920,0:27:37.950
|
|
or if you actually tried to service
|
|
users you could boot a time sharing batch thing
|
|
|
|
0:27:37.950,0:27:40.790
|
|
that would read card images and print
|
|
stuff out
|
|
|
|
0:27:40.790,0:27:44.460
|
|
or they even had TSO the Time Sharing
|
|
Option where you could interactively sit
|
|
|
|
0:27:44.460,0:27:45.559
|
|
and type and send
|
|
|
|
0:27:45.559,0:27:47.560
|
|
stuffs in and get answers back
|
|
|
|
0:27:47.560,0:27:48.570
|
|
and
|
|
|
|
0:27:48.570,0:27:51.429
|
|
also you could boot TSO so whatever set
|
|
of
|
|
|
|
0:27:51.429,0:27:52.219
|
|
|
|
|
|
0:27:52.219,0:27:55.339
|
|
things you need you could boot them and they ran
|
|
independently as if they were running on their
|
|
|
|
0:27:55.339,0:27:56.470
|
|
own machine
|
|
|
|
0:27:56.470,0:28:03.150
|
|
but all the VM did was it give you an exact
|
|
raw copy of the hardware
|
|
|
|
0:28:03.150,0:28:04.529
|
|
so when UNIX came along
|
|
|
|
0:28:04.529,0:28:07.350
|
|
they sort of liked the notion of
|
|
|
|
0:28:07.350,0:28:11.509
|
|
providing the concept of independent
|
|
things that you could operate in
|
|
|
|
0:28:11.509,0:28:13.610
|
|
but they wanted it at a higher level
|
|
|
|
0:28:13.610,0:28:15.610
|
|
so you're looking really to do it
|
|
|
|
0:28:15.610,0:28:17.480
|
|
instead of at the raw hardware level
|
|
|
|
0:28:17.480,0:28:19.679
|
|
to do it at a process level
|
|
|
|
0:28:19.679,0:28:23.799
|
|
and the idea that then was that the interface you
|
|
would program to would be what we think of as
|
|
|
|
0:28:23.799,0:28:26.090
|
|
a System call interface today
|
|
|
|
0:28:26.090,0:28:27.849
|
|
and the idea then was that
|
|
|
|
0:28:27.849,0:28:30.740
|
|
you would be given a process or set of processes
|
|
|
|
0:28:30.740,0:28:34.990
|
|
and those were independent. your process
|
|
couldn't affect
|
|
|
|
0:28:34.990,0:28:38.830
|
|
the address space of another processor. You couldn't reach
|
|
over and mess around with their addresses,
|
|
|
|
0:28:38.830,0:28:41.030
|
|
you couldn't mess around with their I/O
|
|
channels
|
|
|
|
0:28:41.030,0:28:43.179
|
|
you could slow them down by
|
|
|
|
0:28:43.179,0:28:44.299
|
|
being a pig but
|
|
|
|
0:28:44.299,0:28:47.980
|
|
that was about the only way that you could affect
|
|
other processes
|
|
|
|
0:28:47.980,0:28:48.480
|
|
and
|
|
|
|
0:28:48.480,0:28:49.830
|
|
so
|
|
|
|
0:28:49.830,0:28:52.669
|
|
what the interfaces that they had there
|
|
|
|
0:28:52.669,0:28:58.660
|
|
was one that had these characteristics
|
|
had a paged virtual address space
|
|
|
|
0:28:58.660,0:29:02.980
|
|
so you didn't have to know as in the old days how much physical
|
|
memory is on the machine and make your application
|
|
|
|
0:29:02.980,0:29:04.740
|
|
fit into that amount of memory
|
|
|
|
0:29:04.740,0:29:07.950
|
|
you just had what looked like a large
|
|
|
|
0:29:07.950,0:29:11.710
|
|
uniform address space even if the underlying
|
|
hardware had segments or some other
|
|
|
|
0:29:11.710,0:29:13.580
|
|
hardware brain damage
|
|
|
|
0:29:13.580,0:29:17.390
|
|
it looked to you like he just had a big uniform
|
|
address space and
|
|
|
|
0:29:17.390,0:29:21.070
|
|
the size of your address space was independent
|
|
of the amount of memory that was on your machine
|
|
|
|
0:29:21.070,0:29:23.900
|
|
your address space couldn't be bigger than amount of
|
|
physical memory
|
|
|
|
0:29:23.900,0:29:26.499
|
|
cause we sort of move pages around underneath
|
|
|
|
0:29:26.499,0:29:29.320
|
|
whatever part address space was actually
|
|
active
|
|
|
|
0:29:29.320,0:29:34.260
|
|
and there's obviously limits to this if
|
|
you are trying to run a 1 gigabyte of
|
|
|
|
0:29:34.260,0:29:35.630
|
|
application on top of
|
|
|
|
0:29:35.630,0:29:37.240
|
|
ten megabytes of memory
|
|
|
|
0:29:37.240,0:29:40.880
|
|
it's probably going to bring new meaning to
|
|
same day service
|
|
|
|
0:29:40.880,0:29:45.519
|
|
but if you're willing to wait long enough it
|
|
will eventually move the pages around and you will
|
|
|
|
0:29:45.519,0:29:49.740
|
|
progress through getting your application run
|
|
|
|
0:29:49.740,0:29:53.890
|
|
another thing was dealing with software
|
|
interrupts
|
|
|
|
0:29:53.890,0:29:55.789
|
|
in the old days
|
|
|
|
0:29:55.789,0:29:58.749
|
|
you had to understand how the hardware worked
|
|
|
|
0:29:58.749,0:30:03.900
|
|
in order to deal with exceptional conditions
|
|
so for example if you did a divide by zero
|
|
|
|
0:30:03.900,0:30:08.170
|
|
the hardware would jump through some
|
|
vector location or
|
|
|
|
0:30:08.170,0:30:08.630
|
|
something
|
|
|
|
0:30:08.630,0:30:12.799
|
|
and you had know how that worked and make
|
|
sure that you had your program
|
|
|
|
0:30:12.799,0:30:16.510
|
|
usually some little bit of assembly language
|
|
set up to deal with that
|
|
|
|
0:30:16.510,0:30:19.870
|
|
and UNIX said let's let's get away
|
|
from the hardware here
|
|
|
|
0:30:19.870,0:30:22.080
|
|
and so they did this thing called signals
|
|
|
|
0:30:22.080,0:30:25.700
|
|
and so they just define a set of the signals is that
|
|
if you do divide by zero
|
|
|
|
0:30:25.700,0:30:29.529
|
|
you simply register a routine you
|
|
want to have called you don't have to know
|
|
|
|
0:30:29.529,0:30:31.220
|
|
how the hardware figured it out
|
|
|
|
0:30:31.220,0:30:36.740
|
|
you just know that that routine is going to get
|
|
called and you can deal with it at that point
|
|
|
|
0:30:36.740,0:30:40.960
|
|
well we got set of timers and counters to keep
|
|
track of what we're doing, this is really more
|
|
|
|
0:30:40.960,0:30:43.490
|
|
for counting than anything else but
|
|
|
|
0:30:43.490,0:30:46.970
|
|
applications may want to have access to that.
|
|
|
|
0:30:46.970,0:30:51.720
|
|
we have a set of identifiers that we're
|
|
going to use for things like accounting,
|
|
|
|
0:30:51.720,0:30:54.830
|
|
protection and scheduling and so on
|
|
|
|
0:30:54.830,0:30:55.820
|
|
and one of the
|
|
|
|
0:30:55.820,0:31:00.320
|
|
the early philosophies of UNIX was to try
|
|
and keep it simple.
|
|
|
|
0:31:00.320,0:31:02.630
|
|
operating systems have gotten very baroque
|
|
|
|
0:31:02.630,0:31:04.490
|
|
in particular the thing that
|
|
|
|
0:31:04.490,0:31:07.350
|
|
pre dated UNIX was a thing called
|
|
Multix
|
|
|
|
0:31:07.350,0:31:12.820
|
|
Multix was was a joint project between
|
|
Honeywell, a big computer manufacturer of the
|
|
|
|
0:31:12.820,0:31:15.740
|
|
time
|
|
|
|
0:31:15.740,0:31:17.129
|
|
AT&T bell laboratories
|
|
|
|
0:31:17.129,0:31:19.750
|
|
the big industrial laboratory at that time
|
|
|
|
0:31:19.750,0:31:21.380
|
|
and MIT
|
|
|
|
0:31:21.380,0:31:23.430
|
|
a big university then and
|
|
|
|
0:31:23.430,0:31:24.690
|
|
still today
|
|
|
|
0:31:24.690,0:31:29.259
|
|
and those three organizations got
|
|
together to try and build this
|
|
|
|
0:31:29.259,0:31:31.400
|
|
time sharing operating system
|
|
|
|
0:31:31.400,0:31:32.280
|
|
and it
|
|
|
|
0:31:32.280,0:31:33.770
|
|
it just got bigger and
|
|
|
|
0:31:33.770,0:31:37.160
|
|
more grandiose and more complex and never
|
|
finished
|
|
|
|
0:31:37.160,0:31:38.979
|
|
because as soon as they sort of see
|
|
|
|
0:31:38.979,0:31:42.709
|
|
oh we know how to do that but we could
|
|
do this other thing too and so then they would tear it
|
|
|
|
0:31:42.709,0:31:43.429
|
|
apart and
|
|
|
|
0:31:43.429,0:31:46.440
|
|
they never really got to something that
|
|
|
|
0:31:46.440,0:31:48.210
|
|
could be put into production
|
|
|
|
0:31:48.210,0:31:49.919
|
|
and so the
|
|
|
|
0:31:49.919,0:31:50.570
|
|
AT&T
|
|
|
|
0:31:50.570,0:31:54.340
|
|
Bell laboratories decided to pull out of
|
|
that project
|
|
|
|
0:31:54.340,0:31:55.940
|
|
and
|
|
|
|
0:31:55.940,0:32:00.000
|
|
the two of the people that had been working on
|
|
that project, Ken Thompson and Dennis Richie
|
|
|
|
0:32:00.000,0:32:04.390
|
|
were sort of bummed because they were now
|
|
back to typing cards and putting them through
|
|
|
|
0:32:04.390,0:32:05.259
|
|
card readers and
|
|
|
|
0:32:05.259,0:32:07.960
|
|
they had gotten used to the idea that you could
|
|
actually
|
|
|
|
0:32:07.960,0:32:11.559
|
|
sit at an ASSR33 teletype and interact
|
|
with your computer
|
|
|
|
0:32:11.559,0:32:13.440
|
|
and so
|
|
|
|
0:32:13.440,0:32:18.230
|
|
they found an old %uh PDP-8 sitting off in
|
|
the corner that had been abandoned
|
|
|
|
0:32:18.230,0:32:22.120
|
|
and started working on this little tiny operating
|
|
system which they called UNIX
|
|
|
|
0:32:22.120,0:32:26.549
|
|
which eventually moved to the PDP-11 and
|
|
became what we have today
|
|
|
|
0:32:26.549,0:32:28.050
|
|
but because it was
|
|
|
|
0:32:28.050,0:32:32.120
|
|
they were coming first of all from Multix
|
|
where everything had been done and
|
|
|
|
0:32:32.120,0:32:34.110
|
|
in great grandiose detail
|
|
|
|
0:32:34.110,0:32:37.549
|
|
and because they're fundamentally were two
|
|
of them working on it and they wanted to get something
|
|
|
|
0:32:37.549,0:32:38.370
|
|
done and
|
|
|
|
0:32:38.370,0:32:40.130
|
|
within a year or so
|
|
|
|
0:32:40.130,0:32:41.529
|
|
one of their philosophies was
|
|
|
|
0:32:41.529,0:32:44.099
|
|
let's find the one way of doing things
|
|
|
|
0:32:44.099,0:32:48.180
|
|
let's not have eight ways from Sunday let's just
|
|
get the one way
|
|
|
|
0:32:48.180,0:32:53.860
|
|
and that's what we will provide. So what is
|
|
the sort of core set of things that we need.
|
|
|
|
0:32:53.860,0:32:58.620
|
|
well first thing is when it comes to identifiers,
|
|
let's not have you know
|
|
|
|
0:32:58.620,0:33:00.430
|
|
eighty thousand different identifiers
|
|
|
|
0:33:00.430,0:33:03.140
|
|
so they came up with process identifiers,
|
|
|
|
0:33:03.140,0:33:09.620
|
|
user identifier and at that time a single group
|
|
identifier and later expanded
|
|
|
|
0:33:09.620,0:33:14.200
|
|
and they used that sort of identifiers for everything
|
|
so its used for counting, used for making
|
|
|
|
0:33:14.200,0:33:17.410
|
|
protection decisions, used for scheduling
|
|
decisions
|
|
|
|
0:33:17.410,0:33:19.470
|
|
and
|
|
|
|
0:33:19.470,0:33:24.279
|
|
again it was the simplicity of thing which
|
|
was what was driving their decision
|
|
|
|
0:33:24.279,0:33:28.840
|
|
but they're really sort of two key ideas
|
|
that they had
|
|
|
|
0:33:28.840,0:33:30.880
|
|
that really made the difference that
|
|
|
|
0:33:30.880,0:33:32.539
|
|
that's what set them up side
|
|
|
|
0:33:32.539,0:33:34.749
|
|
from what everybody else had done before them
|
|
|
|
0:33:34.749,0:33:35.450
|
|
and which
|
|
|
|
0:33:35.450,0:33:39.740
|
|
in retrospect is something that has been pervasive
|
|
more or less ever since
|
|
|
|
0:33:39.740,0:33:41.869
|
|
the first of these was the notion
|
|
|
|
0:33:41.869,0:33:44.840
|
|
that we have a unique descriptor space
|
|
|
|
0:33:44.840,0:33:46.289
|
|
that is
|
|
|
|
0:33:46.289,0:33:51.250
|
|
given a descriptor it can reference
|
|
any I/O device
|
|
|
|
0:33:51.250,0:33:53.650
|
|
so or even any kind of I/O channel
|
|
|
|
0:33:53.650,0:33:58.270
|
|
so you can have a descriptor for terminal
|
|
or descriptor for a file or descriptive for
|
|
|
|
0:33:58.270,0:34:02.240
|
|
a disk or descriptor for a pipe or descriptor
|
|
for a socket
|
|
|
|
0:34:02.240,0:34:03.500
|
|
and
|
|
|
|
0:34:03.500,0:34:04.790
|
|
you don't need to know
|
|
|
|
0:34:04.790,0:34:07.940
|
|
what it references in order to be able to read
|
|
and write that thing
|
|
|
|
0:34:07.940,0:34:11.290
|
|
so if I hand you a descriptor
|
|
you can read from that the descriptor or you can write
|
|
|
|
0:34:11.290,0:34:13.259
|
|
to that descriptor
|
|
|
|
0:34:13.259,0:34:15.189
|
|
and
|
|
|
|
0:34:15.189,0:34:17.359
|
|
the correct thing will happen
|
|
|
|
0:34:17.359,0:34:19.089
|
|
and you'd say well
|
|
|
|
0:34:19.089,0:34:23.629
|
|
that's so obvious I mean how else could you
|
|
possibly think of doing it?
|
|
|
|
0:34:23.629,0:34:25.179
|
|
well predating UNIX
|
|
|
|
0:34:25.179,0:34:28.059
|
|
everything was done with
|
|
|
|
0:34:28.059,0:34:29.379
|
|
a little subsystem
|
|
|
|
0:34:29.379,0:34:33.419
|
|
that would open a file, read a file, write a
|
|
file, close a file
|
|
|
|
0:34:33.419,0:34:37.429
|
|
and there was another set of system calls which
|
|
would open a terminal, read a terminal, write terminal,
|
|
|
|
0:34:37.429,0:34:38.089
|
|
close terminal
|
|
|
|
0:34:38.089,0:34:39.210
|
|
and yet another one
|
|
|
|
0:34:39.210,0:34:42.409
|
|
which was create a pipe, read a pipe,
|
|
write a pipe and so on.
|
|
|
|
0:34:42.409,0:34:47.699
|
|
so if you are just a drop dead stupid
|
|
program like say CAD
|
|
|
|
0:34:47.699,0:34:51.579
|
|
you would have to have code in there and say was
|
|
my input a terminal which in case I need to
|
|
|
|
0:34:51.579,0:34:53.159
|
|
use the read terminal
|
|
|
|
0:34:53.159,0:34:57.419
|
|
or is it a file which in case I need
|
|
to use read file or is it a pipe in which in case
|
|
|
|
0:34:57.419,0:34:59.189
|
|
I need to use read pipe
|
|
|
|
0:34:59.189,0:35:01.860
|
|
and so the program itself had to have all
|
|
this
|
|
|
|
0:35:01.860,0:35:02.859
|
|
coding in it
|
|
|
|
0:35:02.859,0:35:04.409
|
|
whereas when they went to
|
|
|
|
0:35:04.409,0:35:07.159
|
|
the uniform descriptor space
|
|
|
|
0:35:07.159,0:35:09.630
|
|
CAD doesn't know it doesn't need to know
|
|
it just says
|
|
|
|
0:35:09.630,0:35:10.819
|
|
read my input,
|
|
|
|
0:35:10.819,0:35:13.979
|
|
write the output
|
|
|
|
0:35:13.979,0:35:17.059
|
|
and it works and we add a new type of descriptor
|
|
|
|
0:35:17.059,0:35:17.600
|
|
and
|
|
|
|
0:35:17.600,0:35:21.700
|
|
CAD just continues to work just as it always
|
|
did.
|
|
|
|
0:35:21.700,0:35:24.199
|
|
So this proved to be a very powerful construct
|
|
|
|
0:35:24.199,0:35:27.019
|
|
and pretty much every operating system after
|
|
UNIX
|
|
|
|
0:35:27.019,0:35:28.659
|
|
did that there's
|
|
|
|
0:35:28.659,0:35:30.210
|
|
one exception of %uh
|
|
|
|
0:35:30.210,0:35:32.549
|
|
large company in the Pacific North-West
|
|
|
|
0:35:32.549,0:35:35.830
|
|
that still has not quite uniform descriptor
|
|
space
|
|
|
|
0:35:35.830,0:35:38.380
|
|
but %uh that's part of their legacy that really
|
|
|
|
0:35:38.380,0:35:39.900
|
|
they're working on that.
|
|
|
|
0:35:39.900,0:35:42.009
|
|
Longhorn will be here.
|
|
|
|
0:35:42.009,0:35:43.939
|
|
and anyway
|
|
|
|
0:35:43.939,0:35:46.190
|
|
this set of facilities then
|
|
|
|
0:35:46.190,0:35:50.150
|
|
makes up the UNIX virtual machine
|
|
|
|
0:35:50.150,0:35:51.559
|
|
and
|
|
|
|
0:35:51.559,0:35:55.559
|
|
in some sense we still see virtual machines
|
|
being used today in fact we're seeing sort
|
|
|
|
0:35:55.559,0:35:56.749
|
|
of a reversion
|
|
|
|
0:35:56.749,0:36:01.429
|
|
back to some of the IBM stuff in things
|
|
like the VMware
|
|
|
|
0:36:01.429,0:36:03.079
|
|
which is
|
|
|
|
0:36:03.079,0:36:07.029
|
|
essentially allow you to go back to booting
|
|
native operating systems again so sort of
|
|
|
|
0:36:07.029,0:36:08.280
|
|
interesting to watch
|
|
|
|
0:36:08.280,0:36:09.060
|
|
that the sort of
|
|
|
|
0:36:09.060,0:36:12.919
|
|
pendulum of back going back and forth
|
|
of what's the correct layer
|
|
|
|
0:36:12.919,0:36:14.609
|
|
for for doing
|
|
|
|
0:36:14.609,0:36:18.890
|
|
virtual machines
|
|
|
|
0:36:18.890,0:36:22.499
|
|
Okay? so far so good?
|
|
|
|
0:36:22.499,0:36:24.719
|
|
all right so I said that there were
|
|
|
|
0:36:24.719,0:36:27.160
|
|
two key ideas that UNIX had
|
|
|
|
0:36:27.160,0:36:30.279
|
|
the first of these being the uniform descriptor
|
|
space
|
|
|
|
0:36:30.279,0:36:35.819
|
|
the second one which was really critical was
|
|
this notion of processes as a commodity
|
|
|
|
0:36:35.819,0:36:37.309
|
|
item
|
|
|
|
0:36:37.309,0:36:40.220
|
|
so here on Page 17 I've tried to lay
|
|
it out
|
|
|
|
0:36:40.220,0:36:41.090
|
|
the
|
|
|
|
0:36:41.090,0:36:44.159
|
|
that the components that make up a process
|
|
|
|
0:36:44.159,0:36:45.759
|
|
and
|
|
|
|
0:36:45.759,0:36:50.359
|
|
what do I really mean when I say a process as
|
|
a commodity item
|
|
|
|
0:36:50.359,0:36:53.650
|
|
okay leading up to
|
|
|
|
0:36:53.650,0:36:54.689
|
|
UNIX
|
|
|
|
0:36:54.689,0:36:56.800
|
|
the systems that pre-dated it,
|
|
|
|
0:36:56.800,0:36:59.200
|
|
processes were these very large
|
|
|
|
0:36:59.200,0:37:02.169
|
|
heavyweight expensive things
|
|
|
|
0:37:02.169,0:37:02.779
|
|
and
|
|
|
|
0:37:02.779,0:37:04.539
|
|
if you look at
|
|
|
|
0:37:04.539,0:37:08.629
|
|
MVS which was the operating system
|
|
that ran on IBM for doing multiple processing
|
|
|
|
0:37:08.629,0:37:10.509
|
|
and
|
|
|
|
0:37:10.509,0:37:13.799
|
|
the system administrator would decide at boot
|
|
time
|
|
|
|
0:37:13.799,0:37:17.019
|
|
what degree of multiprocessing they wish
|
|
to support
|
|
|
|
0:37:17.019,0:37:18.140
|
|
so they'd say well
|
|
|
|
0:37:18.140,0:37:20.739
|
|
well, we'll let upto six things happen at once
|
|
|
|
0:37:20.739,0:37:22.490
|
|
and so as part of booting up
|
|
|
|
0:37:22.490,0:37:24.419
|
|
they would create six
|
|
|
|
0:37:24.419,0:37:25.349
|
|
processes
|
|
|
|
0:37:25.349,0:37:30.059
|
|
and now you as a user if you wanted to do
|
|
something let's say you wanted to
|
|
|
|
0:37:30.059,0:37:32.009
|
|
compile and run a program
|
|
|
|
0:37:32.009,0:37:34.960
|
|
you would be given a process
|
|
|
|
0:37:34.960,0:37:36.019
|
|
and it was up to you
|
|
|
|
0:37:36.019,0:37:39.369
|
|
to figure out how to stage what you needed
|
|
done
|
|
|
|
0:37:39.369,0:37:39.819
|
|
and
|
|
|
|
0:37:39.819,0:37:43.930
|
|
that this was often fairly complex
|
|
|
|
0:37:43.930,0:37:47.880
|
|
and so you would have to write out all the
|
|
steps that you wanted
|
|
|
|
0:37:47.880,0:37:50.300
|
|
in this wonderful thing called JCL
|
|
|
|
0:37:50.300,0:37:52.259
|
|
Job Control Language.
|
|
|
|
0:37:52.259,0:37:56.650
|
|
Job Control Language was send mail configuration
|
|
file of the sixties
|
|
|
|
0:37:56.650,0:38:00.679
|
|
there where people whose sole job at the company
|
|
was how to put this stuff together 'cause
|
|
|
|
0:38:00.679,0:38:04.189
|
|
all you had to do is get one extra space or
|
|
a missing comma
|
|
|
|
0:38:04.189,0:38:05.000
|
|
something in there
|
|
|
|
0:38:05.000,0:38:08.630
|
|
and the whole thing would just blow up. it would
|
|
just sort of spit the card deck back at
|
|
|
|
0:38:08.630,0:38:09.799
|
|
you and say well
|
|
|
|
0:38:09.799,0:38:13.500
|
|
somewhere in there is a mistake that's sort of
|
|
in the general area of this card
|
|
|
|
0:38:13.500,0:38:15.549
|
|
and I can't deal with it. Fix it.
|
|
|
|
0:38:15.549,0:38:16.489
|
|
and of course
|
|
|
|
0:38:16.489,0:38:20.550
|
|
in those days it wasn't just a matter of hitting
|
|
carriage when you know make carriage return you have to
|
|
|
|
0:38:20.550,0:38:25.239
|
|
get your deck pull out the card, and type the
|
|
new one, put it back in and re-submit it
|
|
|
|
0:38:25.239,0:38:28.729
|
|
As heaven forbid you couldn't touch that
|
|
card reader you know, it had to be done by
|
|
|
|
0:38:28.729,0:38:29.970
|
|
an operator
|
|
|
|
0:38:29.970,0:38:32.869
|
|
so the card deck will read through it would
|
|
disappear and
|
|
|
|
0:38:32.869,0:38:36.800
|
|
you know if you're lucky a few minutes later
|
|
if you were not lucky a few hours later
|
|
|
|
0:38:36.800,0:38:37.849
|
|
you would get
|
|
|
|
0:38:37.849,0:38:39.570
|
|
a print out
|
|
|
|
0:38:39.570,0:38:43.419
|
|
which was what had happened and then you could
|
|
look at it and you know
|
|
|
|
0:38:43.419,0:38:47.209
|
|
I put a comma in the wrong place I guess
|
|
I get to do it all again
|
|
|
|
0:38:47.209,0:38:49.930
|
|
so
|
|
|
|
0:38:49.930,0:38:54.940
|
|
the thing you would need to do there for compiling and running a program
|
|
|
|
0:38:54.940,0:38:59.579
|
|
was you'd have to break into these steps. well
|
|
I need to run the preprocessor
|
|
|
|
0:38:59.579,0:39:04.670
|
|
and so clean out whatever gump that was left
|
|
over on that process from the previous user
|
|
|
|
0:39:04.670,0:39:06.240
|
|
put the preprocessor in there
|
|
|
|
0:39:06.240,0:39:10.530
|
|
and then read from this file here let's
|
|
say I gotta put it somewhere so creative
|
|
|
|
0:39:10.530,0:39:12.510
|
|
scratch file over on this disk and
|
|
|
|
0:39:12.510,0:39:17.299
|
|
it was excruciating detail like how many cylinders
|
|
and how many tracks and this and that
|
|
|
|
0:39:17.299,0:39:19.139
|
|
blocks blah blah blah
|
|
|
|
0:39:19.139,0:39:23.119
|
|
and don't forget any of those parameters 'cause
|
|
it'll spit it out if you do
|
|
|
|
0:39:23.119,0:39:26.890
|
|
and so then it would run the first step in that
|
|
if its successful then you'd have sitting
|
|
|
|
0:39:26.890,0:39:28.899
|
|
in this scratch file that you had created
|
|
|
|
0:39:28.899,0:39:33.100
|
|
the output of the preprocessor and then
|
|
you'd load the first pass of the compiler
|
|
|
|
0:39:33.100,0:39:36.930
|
|
and you say now read from that scratch file
|
|
and create this other scratch file over here and
|
|
|
|
0:39:36.930,0:39:39.450
|
|
when thats successful and we need to delete that
|
|
one
|
|
|
|
0:39:39.450,0:39:43.830
|
|
and then load the second pass, put that back
|
|
into another scratch file and then we run this
|
|
|
|
0:39:43.830,0:39:45.950
|
|
assembler, and the optimizer then the
|
|
|
|
0:39:45.950,0:39:47.750
|
|
loader this and that
|
|
|
|
0:39:47.750,0:39:49.410
|
|
finally run the program
|
|
|
|
0:39:49.410,0:39:50.900
|
|
and if all goes well
|
|
|
|
0:39:50.900,0:39:57.029
|
|
you know at step sixteen out comes the answer
|
|
|
|
0:39:57.029,0:39:58.129
|
|
forty two. so UNIX
|
|
|
|
0:39:58.129,0:40:00.819
|
|
said, look this is silly
|
|
|
|
0:40:00.819,0:40:02.880
|
|
a lot of this is just
|
|
|
|
0:40:02.880,0:40:04.310
|
|
bookkeeping
|
|
|
|
0:40:04.310,0:40:07.249
|
|
and computers do bookkeeping really well
|
|
|
|
0:40:07.249,0:40:12.179
|
|
and you'll recall yeah but it's going to take
|
|
all these cycles it's like
|
|
|
|
0:40:12.179,0:40:16.309
|
|
computers are supposed to be labor-saving
|
|
devices right? so
|
|
|
|
0:40:16.309,0:40:20.150
|
|
they came up with this notion that they would
|
|
create processes on the fly as needed
|
|
|
|
0:40:20.150,0:40:21.159
|
|
you had
|
|
|
|
0:40:21.159,0:40:25.549
|
|
you've had a preprocessor in two
|
|
steps of the compiler and then
|
|
|
|
0:40:25.549,0:40:27.109
|
|
optimizer and then a loader
|
|
|
|
0:40:27.109,0:40:29.410
|
|
we just create Boom seven processes
|
|
|
|
0:40:29.410,0:40:31.920
|
|
and we connect them together with pipes
|
|
|
|
0:40:31.920,0:40:35.180
|
|
and so we take the input and you know run
|
|
through in
|
|
|
|
0:40:35.180,0:40:38.270
|
|
through the pipes and you know out the end
|
|
you get the the
|
|
|
|
0:40:38.270,0:40:39.629
|
|
executable
|
|
|
|
0:40:39.629,0:40:40.030
|
|
and
|
|
|
|
0:40:40.030,0:40:42.880
|
|
we will simply create each of these processes
|
|
|
|
0:40:42.880,0:40:44.650
|
|
and
|
|
|
|
0:40:44.650,0:40:46.549
|
|
so you as a user just
|
|
|
|
0:40:46.549,0:40:49.479
|
|
type you know the C compiler and it just
|
|
|
|
0:40:49.479,0:40:52.429
|
|
fork these things pipe them together got the result
|
|
|
|
0:40:52.429,0:40:53.640
|
|
and
|
|
|
|
0:40:53.640,0:40:57.509
|
|
then once it was done with this processes is
|
|
just threw them away so any time you'd create a
|
|
|
|
0:40:57.509,0:41:00.479
|
|
new process and it came to you pristine clean
|
|
|
|
0:41:00.479,0:41:04.239
|
|
and you needed a bunch of things it did
|
|
put everything in intermediate files
|
|
|
|
0:41:04.239,0:41:07.549
|
|
the fact of the matter is in the early days
|
|
|
|
0:41:07.549,0:41:08.129
|
|
those computers
|
|
|
|
0:41:08.129,0:41:11.910
|
|
didn't really have enough memory to support
|
|
all that stuff at once so
|
|
|
|
0:41:11.910,0:41:15.809
|
|
behind you those pipes were actually implemented
|
|
as files
|
|
|
|
0:41:15.809,0:41:19.319
|
|
but you didn't have at least to remember to create
|
|
them and delete them
|
|
|
|
0:41:19.319,0:41:20.200
|
|
and deal with them
|
|
|
|
0:41:20.200,0:41:24.020
|
|
as far as you were concerned it just look stuff
|
|
flowing through pipes and of course today it
|
|
|
|
0:41:24.020,0:41:24.490
|
|
just
|
|
|
|
0:41:24.490,0:41:27.989
|
|
does flow through pipes in memory
|
|
|
|
0:41:27.989,0:41:29.439
|
|
okay so
|
|
|
|
0:41:29.439,0:41:33.689
|
|
this notion then that that we're just gonna
|
|
create processes on the fly is needed and
|
|
|
|
0:41:33.689,0:41:35.559
|
|
connect them together as needed
|
|
|
|
0:41:35.559,0:41:38.039
|
|
it was a novel concept
|
|
|
|
0:41:38.039,0:41:43.599
|
|
and it wasn't that somehow mysteriously figured
|
|
out how to create processes cheaply
|
|
|
|
0:41:43.599,0:41:44.839
|
|
cause they hadn't
|
|
|
|
0:41:44.839,0:41:46.180
|
|
they were still
|
|
|
|
0:41:46.180,0:41:49.959
|
|
really expensive to create
|
|
|
|
0:41:49.959,0:41:52.210
|
|
but that extra effort
|
|
|
|
0:41:52.210,0:41:53.029
|
|
was
|
|
|
|
0:41:53.029,0:41:56.089
|
|
worth it because it was saving a lot of programming
|
|
time
|
|
|
|
0:41:56.089,0:41:59.809
|
|
so my favorite example is you run ls
|
|
|
|
0:41:59.809,0:42:01.810
|
|
so we have to create a process
|
|
|
|
0:42:01.810,0:42:04.259
|
|
load the ls binary into it
|
|
|
|
0:42:04.259,0:42:06.180
|
|
it prints a line or two on your screen
|
|
|
|
0:42:06.180,0:42:10.609
|
|
and we tear the entire thing down and return
|
|
all its resources back to the system
|
|
|
|
0:42:10.609,0:42:14.979
|
|
more than ninety percent of the cost of running
|
|
ls is creating and destroying the process
|
|
|
|
0:42:14.979,0:42:19.239
|
|
a tiny fraction of it is actually running
|
|
ls
|
|
|
|
0:42:19.239,0:42:24.259
|
|
but it goes so fast, who cares right
|
|
|
|
0:42:24.259,0:42:25.749
|
|
so the point is that
|
|
|
|
0:42:25.749,0:42:30.039
|
|
that concept of just creating things as
|
|
needed
|
|
|
|
0:42:30.039,0:42:31.780
|
|
again was very powerful
|
|
|
|
0:42:31.780,0:42:35.709
|
|
and is one that is just pervasive today
|
|
|
|
0:42:35.709,0:42:38.639
|
|
okay so what is a process actually made up
|
|
of
|
|
|
|
0:42:38.639,0:42:43.179
|
|
it gets some amount of CPU time or at
|
|
least we do dearly hope that it gets some
|
|
|
|
0:42:43.179,0:42:46.050
|
|
amount of CPU time, the lack of getting
|
|
CPU time
|
|
|
|
0:42:46.050,0:42:46.670
|
|
that makes it
|
|
|
|
0:42:46.670,0:42:47.979
|
|
a computer so sluggish
|
|
|
|
0:42:47.979,0:42:49.409
|
|
of course
|
|
|
|
0:42:49.409,0:42:51.920
|
|
others really boils down to scheduling
|
|
|
|
0:42:51.920,0:42:54.249
|
|
and we're going to talk about scheduling
|
|
|
|
0:42:54.249,0:42:56.279
|
|
probably more than you care to
|
|
|
|
0:42:56.279,0:42:59.219
|
|
in a couple weeks time
|
|
|
|
0:42:59.219,0:43:01.619
|
|
we have the asynchronous events
|
|
|
|
0:43:01.619,0:43:04.569
|
|
these are the external events that
|
|
|
|
0:43:04.569,0:43:05.659
|
|
are coming in
|
|
|
|
0:43:05.659,0:43:07.679
|
|
so
|
|
|
|
0:43:07.679,0:43:10.169
|
|
they may be either things that
|
|
|
|
0:43:10.169,0:43:14.339
|
|
were coming in from the outside world like
|
|
start, stop and quit
|
|
|
|
0:43:14.339,0:43:15.279
|
|
oh
|
|
|
|
0:43:15.279,0:43:18.170
|
|
out-of-band data arrival notification that kind
|
|
of thing
|
|
|
|
0:43:18.170,0:43:22.339
|
|
or it may in fact be things that the program
|
|
is bringing down upon itself
|
|
|
|
0:43:22.339,0:43:25.590
|
|
such as a segment fault, a divide by zero
|
|
|
|
0:43:25.590,0:43:26.910
|
|
and some other
|
|
|
|
0:43:26.910,0:43:31.959
|
|
what would normally be viewed as incorrect
|
|
operation
|
|
|
|
0:43:31.959,0:43:35.849
|
|
and so we'll talk about that when we talk about
|
|
signals
|
|
|
|
0:43:35.849,0:43:37.039
|
|
every program
|
|
|
|
0:43:37.039,0:43:38.899
|
|
gets some amount of memory
|
|
|
|
0:43:38.899,0:43:42.659
|
|
it gets an initial amount when it starts
|
|
up injured generally allocates more as it
|
|
|
|
0:43:42.659,0:43:45.229
|
|
goes along
|
|
|
|
0:43:45.229,0:43:49.429
|
|
this of course we will deal with very extensively
|
|
will spend an entire week on it
|
|
|
|
0:43:49.429,0:43:54.249
|
|
when we talk about how virtual memory is implemented
|
|
|
|
0:43:54.249,0:43:54.609
|
|
and
|
|
|
|
0:43:54.609,0:43:57.429
|
|
then we get I/O descriptors
|
|
|
|
0:43:57.429,0:44:02.259
|
|
I used to say that every program had to have
|
|
at least one I/O descriptor since
|
|
|
|
0:44:02.259,0:44:04.910
|
|
it absolutely had no input
|
|
|
|
0:44:04.910,0:44:06.329
|
|
absolutely no output
|
|
|
|
0:44:06.329,0:44:09.049
|
|
then it was sort of pointless
|
|
|
|
0:44:09.049,0:44:12.900
|
|
of course I had to have one of my students
|
|
come up and point out to me there is an a
|
|
|
|
0:44:12.900,0:44:13.849
|
|
class of programs
|
|
|
|
0:44:13.849,0:44:16.469
|
|
which don't need I/O descriptors
|
|
|
|
0:44:16.469,0:44:17.440
|
|
and that is
|
|
|
|
0:44:17.440,0:44:19.549
|
|
these things called benchmarks
|
|
|
|
0:44:19.549,0:44:23.249
|
|
it just compute something all we really care
|
|
about is how long it takes them to compute
|
|
|
|
0:44:23.249,0:44:24.959
|
|
we don't actually care what the answer is
|
|
|
|
0:44:24.959,0:44:26.019
|
|
In theory we don't
|
|
|
|
0:44:26.019,0:44:29.779
|
|
I personally like my benchmark stop with
|
|
something so I can see it there
|
|
|
|
0:44:29.779,0:44:31.489
|
|
doing computing the right thing
|
|
|
|
0:44:31.489,0:44:33.169
|
|
but in theory
|
|
|
|
0:44:33.169,0:44:35.919
|
|
that wouldn't be necessary
|
|
|
|
0:44:35.919,0:44:38.650
|
|
outside of that class of programs
|
|
|
|
0:44:38.650,0:44:42.670
|
|
everything needs some sort of descriptors and
|
|
of course we'll talk about descriptors
|
|
|
|
0:44:42.670,0:44:43.659
|
|
quite extensively
|
|
|
|
0:44:43.659,0:44:47.349
|
|
as we go through the I/O subsystem
|
|
|
|
0:44:47.349,0:44:50.969
|
|
okay so the executive summary is that processes
|
|
are
|
|
|
|
0:44:50.969,0:44:54.969
|
|
the fundamental service that is provided by
|
|
UNIX
|
|
|
|
0:44:54.969,0:44:58.430
|
|
and
|
|
|
|
0:44:58.430,0:45:02.849
|
|
what we're going to spend essentially the
|
|
next two and a half weeks working on
|
|
|
|
0:45:02.849,0:45:04.769
|
|
is
|
|
|
|
0:45:04.769,0:45:07.079
|
|
what what makes up processes
|
|
|
|
0:45:07.079,0:45:10.180
|
|
we'll go into much more detail about each of these
|
|
four points
|
|
|
|
0:45:10.180,0:45:11.769
|
|
and
|
|
|
|
0:45:11.769,0:45:13.630
|
|
then how do we actually go about
|
|
|
|
0:45:13.630,0:45:14.390
|
|
providing that
|
|
|
|
0:45:14.390,0:45:16.639
|
|
bit of service
|
|
|
|
0:45:16.639,0:45:17.900
|
|
the next thing that I'm
|
|
|
|
0:45:17.900,0:45:22.210
|
|
going to do now is this go through and lay
|
|
out some of the terminology that
|
|
|
|
0:45:22.210,0:45:23.239
|
|
we have when
|
|
|
|
0:45:23.239,0:45:25.130
|
|
we're talking about processes
|
|
|
|
0:45:25.130,0:45:29.229
|
|
so this is sort of the big picture here were
|
|
on page eighteen
|
|
|
|
0:45:29.229,0:45:30.669
|
|
and
|
|
|
|
0:45:30.669,0:45:33.669
|
|
you can see we have sort of three bits that
|
|
make up
|
|
|
|
0:45:33.669,0:45:36.640
|
|
the system
|
|
|
|
0:45:36.640,0:45:39.029
|
|
we have the currently running user process
|
|
|
|
0:45:39.029,0:45:41.180
|
|
and then what we call the top half of the kernel
|
|
|
|
0:45:41.180,0:45:43.699
|
|
and the bottom half of the kernel
|
|
|
|
0:45:43.699,0:45:47.049
|
|
now this would be a picture for a uniprocessor
|
|
|
|
0:45:47.049,0:45:49.299
|
|
so one CPU
|
|
|
|
0:45:49.299,0:45:51.209
|
|
if we had a multiprocessor
|
|
|
|
0:45:51.209,0:45:54.009
|
|
%uh then we would have
|
|
|
|
0:45:54.009,0:45:57.130
|
|
one instance of the kernel
|
|
|
|
0:45:57.130,0:45:59.529
|
|
but multiple instances of the user process
|
|
|
|
0:45:59.529,0:46:02.879
|
|
but for any given CPU on a multiprocessor
|
|
|
|
0:46:02.879,0:46:05.709
|
|
it is running exactly one process
|
|
|
|
0:46:05.709,0:46:09.309
|
|
so you may think they we're running for four-five
|
|
processes all at once
|
|
|
|
0:46:09.309,0:46:14.319
|
|
but the fact of the matter is that any instant
|
|
in time there's only one process which is
|
|
|
|
0:46:14.319,0:46:16.299
|
|
actually running
|
|
|
|
0:46:16.299,0:46:18.609
|
|
and
|
|
|
|
0:46:18.609,0:46:21.429
|
|
that is the one that we have loaded in the system
|
|
|
|
0:46:21.429,0:46:25.199
|
|
now we give the illusion that were running
|
|
lots of things because we switch between them
|
|
|
|
0:46:25.199,0:46:26.100
|
|
rather quickly
|
|
|
|
0:46:26.100,0:46:29.269
|
|
so it looks like things are happening in all
|
|
windows at once
|
|
|
|
0:46:29.269,0:46:31.430
|
|
but in reality
|
|
|
|
0:46:31.430,0:46:33.619
|
|
that's not really happening
|
|
|
|
0:46:33.619,0:46:36.440
|
|
okay so there is a set of properties that I want to
|
|
look at
|
|
|
|
0:46:36.440,0:46:40.899
|
|
that had to do with each one of these parts here
|
|
|
|
0:46:40.899,0:46:44.359
|
|
but just to sort of look at it from the
|
|
big picture perspective
|
|
|
|
0:46:44.359,0:46:45.970
|
|
what you see here
|
|
|
|
0:46:45.970,0:46:47.180
|
|
is
|
|
|
|
0:46:47.180,0:46:51.549
|
|
there is boundary between the user process
|
|
and the top half of the kernel
|
|
|
|
0:46:51.549,0:46:54.949
|
|
which is really just like a glorified sovereignty
|
|
call
|
|
|
|
0:46:54.949,0:46:59.539
|
|
it's a lot like calling into a library routine
|
|
like calling strcat, strcpy or something
|
|
|
|
0:46:59.539,0:47:00.319
|
|
like that
|
|
|
|
0:47:00.319,0:47:03.679
|
|
when you do a system call
|
|
|
|
0:47:03.679,0:47:05.650
|
|
we take that same set of parameters
|
|
|
|
0:47:05.650,0:47:08.009
|
|
now this is sort of
|
|
|
|
0:47:08.009,0:47:09.780
|
|
brick Wall here if you will
|
|
|
|
0:47:09.780,0:47:11.380
|
|
that is protecting
|
|
|
|
0:47:11.380,0:47:13.680
|
|
the top half of the kernel
|
|
|
|
0:47:13.680,0:47:15.299
|
|
from the application
|
|
|
|
0:47:15.299,0:47:18.899
|
|
I'll go more into some detail about how that
|
|
actually gets implemented
|
|
|
|
0:47:18.899,0:47:22.729
|
|
but in essence you can think of it
|
|
is is there sort of this whaling Wall and these little
|
|
|
|
0:47:22.729,0:47:24.990
|
|
chinks there and you can sort of push a request
|
|
through
|
|
|
|
0:47:24.990,0:47:28.230
|
|
and somebody other sides sort of pulls that
|
|
looks at it and decides whether they're going
|
|
|
|
0:47:28.230,0:47:28.690
|
|
to
|
|
|
|
0:47:28.690,0:47:30.769
|
|
dain to provide service to you
|
|
|
|
0:47:30.769,0:47:34.229
|
|
and if they do then they sort of send it back
|
|
|
|
0:47:34.229,0:47:37.649
|
|
well like a library where you can just sort
|
|
of reach in and walk around if you want to
|
|
|
|
0:47:37.649,0:47:38.290
|
|
you
|
|
|
|
0:47:38.290,0:47:40.950
|
|
good programming practices you don't do that
|
|
but
|
|
|
|
0:47:40.950,0:47:43.049
|
|
you could
|
|
|
|
0:47:43.049,0:47:44.579
|
|
all right so
|
|
|
|
0:47:44.579,0:47:49.089
|
|
the the top half of the kernel is really looks
|
|
a lot like
|
|
|
|
0:47:49.089,0:47:50.509
|
|
a big library
|
|
|
|
0:47:50.509,0:47:53.509
|
|
%uh it just happens to be a library
|
|
routines
|
|
|
|
0:47:53.509,0:47:57.599
|
|
that deal with things where processes need
|
|
to interact with each other
|
|
|
|
0:47:57.599,0:48:01.399
|
|
in fact for many people they don't understand
|
|
for what's the difference between the C
|
|
|
|
0:48:01.399,0:48:03.259
|
|
library and the top half of the kernel
|
|
|
|
0:48:03.259,0:48:08.020
|
|
if it's something that you're doing that
|
|
no other process needs to know about
|
|
|
|
0:48:08.020,0:48:09.799
|
|
then it can be in the C library
|
|
|
|
0:48:09.799,0:48:13.829
|
|
so if you call strcat to concatenate two
|
|
strings together
|
|
|
|
0:48:13.829,0:48:17.599
|
|
nobody else needs to know you're doing that
|
|
you don't need to coordinate with anybody
|
|
|
|
0:48:17.599,0:48:19.000
|
|
else that you're doing that
|
|
|
|
0:48:19.000,0:48:20.160
|
|
it's just happening
|
|
|
|
0:48:20.160,0:48:21.979
|
|
so that goes in the C library.
|
|
|
|
0:48:21.979,0:48:24.489
|
|
on the other hand if you're reading or writing
|
|
the file
|
|
|
|
0:48:24.489,0:48:28.029
|
|
there may be other processes that are also
|
|
reading and writing that file
|
|
|
|
0:48:28.029,0:48:29.910
|
|
and therefore that
|
|
|
|
0:48:29.910,0:48:31.579
|
|
has to be done by the kernel
|
|
|
|
0:48:31.579,0:48:33.120
|
|
because they can coordinate
|
|
|
|
0:48:33.120,0:48:37.189
|
|
all the different processes that are trying to access
|
|
that file.
|
|
|
|
0:48:37.189,0:48:40.529
|
|
so the top half of the kernel is pretty straightforward
|
|
code
|
|
|
|
0:48:40.529,0:48:45.539
|
|
it looks a lot like any other library that
|
|
you would write if you look at top half kernel
|
|
|
|
0:48:45.539,0:48:49.640
|
|
code you know you see all read, come in
|
|
it's got these parameters we Mark around we
|
|
|
|
0:48:49.640,0:48:53.719
|
|
get some data that we put it in the buffer and
|
|
we return back
|
|
|
|
0:48:53.719,0:48:57.470
|
|
and in fact writing code for the top half of
|
|
the kernel is
|
|
|
|
0:48:57.470,0:48:59.729
|
|
not all that difficult to do
|
|
|
|
0:48:59.729,0:49:00.989
|
|
it's
|
|
|
|
0:49:00.989,0:49:01.959
|
|
you have
|
|
|
|
0:49:01.959,0:49:05.939
|
|
for many of the same properties that you would
|
|
when you're writing user level application
|
|
|
|
0:49:05.939,0:49:07.529
|
|
code
|
|
|
|
0:49:07.529,0:49:11.779
|
|
the bottom half of the kernel is where things
|
|
start to get nasty
|
|
|
|
0:49:11.779,0:49:14.820
|
|
because the bottom half of the kernel is the part
|
|
of the system
|
|
|
|
0:49:14.820,0:49:18.769
|
|
that deals with all of the asynchronous events
|
|
in the system
|
|
|
|
0:49:18.769,0:49:22.179
|
|
is things like device drivers,
|
|
|
|
0:49:22.179,0:49:23.779
|
|
timers
|
|
|
|
0:49:23.779,0:49:25.010
|
|
that level of thing
|
|
|
|
0:49:25.010,0:49:28.029
|
|
that are driven by hardware events
|
|
|
|
0:49:28.029,0:49:28.659
|
|
so
|
|
|
|
0:49:28.659,0:49:31.459
|
|
for example a packet arrives on the network
|
|
|
|
0:49:31.459,0:49:33.670
|
|
that causes an interrupt to come and
|
|
|
|
0:49:33.670,0:49:36.729
|
|
that will be handled by the bottom half of
|
|
the kernel
|
|
|
|
0:49:36.729,0:49:38.829
|
|
and historically
|
|
|
|
0:49:38.829,0:49:43.079
|
|
when an interrupt came in it preempted whatever
|
|
else was going on
|
|
|
|
0:49:43.079,0:49:45.400
|
|
and it ran until it finished and then it returned
|
|
|
|
0:49:45.400,0:49:46.539
|
|
and it could not
|
|
|
|
0:49:46.539,0:49:49.439
|
|
go to sleep to wait for resources or other
|
|
things
|
|
|
|
0:49:49.439,0:49:51.339
|
|
%uh in current systems
|
|
|
|
0:49:51.339,0:49:54.549
|
|
you can actually go to sleep in the interrupt driver
|
|
and waiting for
|
|
|
|
0:49:54.549,0:49:56.739
|
|
some other activity to complete
|
|
|
|
0:49:56.739,0:49:58.259
|
|
it is however
|
|
|
|
0:49:58.259,0:50:00.799
|
|
not a good idea to do that
|
|
|
|
0:50:00.799,0:50:01.909
|
|
because
|
|
|
|
0:50:01.909,0:50:06.739
|
|
the usual case of most device drivers is they
|
|
can finish whatever they're doing in an interrupt
|
|
|
|
0:50:06.739,0:50:08.579
|
|
without ever blocking
|
|
|
|
0:50:08.579,0:50:09.580
|
|
and so
|
|
|
|
0:50:09.580,0:50:13.649
|
|
when an interrupt comes in we assume that you're
|
|
not going to sleep
|
|
|
|
0:50:13.649,0:50:14.710
|
|
and if you actually
|
|
|
|
0:50:14.710,0:50:17.219
|
|
then go to sleep.oh man
|
|
|
|
0:50:17.219,0:50:20.469
|
|
you didn't tell us you're going to do this we
|
|
have to go off to do a whole lot of other work
|
|
|
|
0:50:20.469,0:50:23.029
|
|
that we had originally planned on doing
|
|
|
|
0:50:23.029,0:50:25.460
|
|
so if you go to sleep in a device driver
|
|
|
|
0:50:25.460,0:50:28.209
|
|
you are taking a very serious performance hit
|
|
|
|
0:50:28.209,0:50:31.019
|
|
so it's highly recommended that you don't
|
|
do that
|
|
|
|
0:50:31.019,0:50:33.130
|
|
but if you have to you can
|
|
|
|
0:50:33.130,0:50:35.809
|
|
on it's because of this historic behavior
|
|
or
|
|
|
|
0:50:35.809,0:50:39.899
|
|
of not being able to sleep in the bottom half
|
|
of the kernel
|
|
|
|
0:50:39.899,0:50:42.119
|
|
that you have certain properties that have
|
|
|
|
0:50:42.119,0:50:44.769
|
|
taken over in device drivers
|
|
|
|
0:50:44.769,0:50:45.940
|
|
and that is
|
|
|
|
0:50:45.940,0:50:50.369
|
|
that a device driver should be handed all
|
|
the resources it needs to get his job done
|
|
|
|
0:50:50.369,0:50:54.490
|
|
you don't give a disk device driver
|
|
Go read this
|
|
|
|
0:50:54.490,0:50:56.549
|
|
and put it somewhere
|
|
|
|
0:50:56.549,0:50:57.580
|
|
you have to say
|
|
|
|
0:50:57.580,0:50:59.410
|
|
Go read this particular block
|
|
|
|
0:50:59.410,0:51:02.650
|
|
here is a chunk of memory that I want that
|
|
data to put in
|
|
|
|
0:51:02.650,0:51:03.959
|
|
and
|
|
|
|
0:51:03.959,0:51:06.169
|
|
notify me when it's done
|
|
|
|
0:51:06.169,0:51:06.970
|
|
because
|
|
|
|
0:51:06.970,0:51:10.660
|
|
things like allocating memory are classic
|
|
places where you end up having to go to sleep
|
|
|
|
0:51:10.660,0:51:12.939
|
|
to wait for stuff to happen
|
|
|
|
0:51:12.939,0:51:14.449
|
|
and
|
|
|
|
0:51:14.449,0:51:16.390
|
|
historically you couldn't do that
|
|
|
|
0:51:16.390,0:51:18.640
|
|
even currently don't want to have to do that
|
|
|
|
0:51:18.640,0:51:23.400
|
|
so device drivers generally have all
|
|
resources pre allocated
|
|
|
|
0:51:23.400,0:51:25.169
|
|
and then they can just go
|
|
|
|
0:51:25.169,0:51:27.279
|
|
the one place where this doesn't work
|
|
|
|
0:51:27.279,0:51:29.029
|
|
is the network
|
|
|
|
0:51:29.029,0:51:30.929
|
|
and in particular
|
|
|
|
0:51:30.929,0:51:34.630
|
|
you don't know when somebody's going to send
|
|
packets to you
|
|
|
|
0:51:34.630,0:51:37.040
|
|
you say well you're looking to open connections
|
|
|
|
0:51:37.040,0:51:39.360
|
|
but if you're doing something like IP forwarding
|
|
|
|
0:51:39.360,0:51:40.969
|
|
there's no
|
|
|
|
0:51:40.969,0:51:45.039
|
|
top half state it's dealing with this packets
|
|
they're just coming in on one interface being
|
|
|
|
0:51:45.039,0:51:46.719
|
|
sent out on another interface
|
|
|
|
0:51:46.719,0:51:50.630
|
|
they never pass through any part of the top
|
|
half of the kernel
|
|
|
|
0:51:50.630,0:51:53.529
|
|
and so in the case of network device drivers
|
|
|
|
0:51:53.529,0:51:56.149
|
|
they need to allocate memory
|
|
|
|
0:51:56.149,0:51:56.640
|
|
and
|
|
|
|
0:51:56.640,0:51:58.829
|
|
if memory gets into short supply
|
|
|
|
0:51:58.829,0:52:01.689
|
|
and they try to allocate memory and it's not
|
|
available
|
|
|
|
0:52:01.689,0:52:05.049
|
|
they historically couldn't wait for memory to be
|
|
available
|
|
|
|
0:52:05.049,0:52:08.380
|
|
and even in practice today don't wait
|
|
|
|
|
|
0:52:08.380,0:52:09.580
|
|
for memory to become available
|
|
|
|
0:52:09.580,0:52:12.469
|
|
they simply drop the packet on the floor
|
|
|
|
0:52:12.469,0:52:18.109
|
|
it's like well I didn't have any place to
|
|
put it sorry oops
|
|
|
|
0:52:18.109,0:52:20.940
|
|
now that doesn't cause incorrect behavior
|
|
|
|
0:52:20.940,0:52:24.369
|
|
because the higher level protocols will retransmit
|
|
|
|
0:52:24.369,0:52:29.140
|
|
but it does cause great performance problems
|
|
because retransmission means that connections
|
|
|
|
0:52:29.140,0:52:29.879
|
|
stall
|
|
|
|
0:52:29.879,0:52:31.110
|
|
they have to back up
|
|
|
|
0:52:31.110,0:52:33.010
|
|
they have to resend data
|
|
|
|
0:52:33.010,0:52:33.739
|
|
and so on
|
|
|
|
0:52:33.739,0:52:38.739
|
|
so you really want to avoid dropping packets
|
|
if you can possibly help it
|
|
|
|
0:52:38.739,0:52:42.029
|
|
and consequently
|
|
|
|
0:52:42.029,0:52:43.420
|
|
we tend to
|
|
|
|
0:52:43.420,0:52:46.499
|
|
pre allocate a certain amount of memory for
|
|
the network drivers
|
|
|
|
0:52:46.499,0:52:48.299
|
|
and
|
|
|
|
0:52:48.299,0:52:52.169
|
|
we try very hard to make sure that we're not
|
|
going to run out of memory but
|
|
|
|
0:52:52.169,0:52:54.869
|
|
if packets come fast enough and we can't deal
|
|
with them
|
|
|
|
0:52:54.869,0:52:57.940
|
|
as quickly as they are arriving then over short period
|
|
of time
|
|
|
|
0:52:57.940,0:53:03.489
|
|
we get to the point where we simply have to start
|
|
dropping packets
|
|
|
|
0:53:03.489,0:53:07.649
|
|
okay this is a part of kernel that you do not wish to
|
|
write code for
|
|
|
|
0:53:07.649,0:53:10.919
|
|
because it is extremely difficult to
|
|
debug
|
|
|
|
0:53:10.919,0:53:12.759
|
|
you get these bugs where
|
|
|
|
0:53:12.759,0:53:18.779
|
|
the only time it happens is on the third Tuesday
|
|
when there's a full moon
|
|
|
|
0:53:18.779,0:53:19.300
|
|
and
|
|
|
|
0:53:19.300,0:53:24.199
|
|
we have a disk interrupt followed by %uh a
|
|
terminal character coming in
|
|
|
|
0:53:24.199,0:53:28.289
|
|
and the network packet arriving of size fifteen
|
|
twenty two
|
|
|
|
0:53:28.289,0:53:30.109
|
|
and when all those things happened
|
|
|
|
0:53:30.109,0:53:32.719
|
|
the system panics
|
|
|
|
0:53:32.719,0:53:37.380
|
|
and of course there's like it panics
|
|
cause you're following some bad pointer
|
|
|
|
0:53:37.380,0:53:40.969
|
|
something that should have been there
|
|
but was freed some time in the distant past
|
|
|
|
0:53:40.969,0:53:42.930
|
|
we are not sure when
|
|
|
|
0:53:42.930,0:53:44.049
|
|
and
|
|
|
|
0:53:44.049,0:53:47.400
|
|
try to debug things like that is extremely
|
|
difficult
|
|
|
|
0:53:47.400,0:53:48.509
|
|
and you can
|
|
|
|
0:53:48.509,0:53:52.120
|
|
think well I think I found the problem but
|
|
it's not reproduceable
|
|
|
|
0:53:52.120,0:53:55.530
|
|
you know you have to wait for the next third
|
|
Tuesday with a full moon and blah blah blah
|
|
|
|
0:53:55.530,0:53:56.950
|
|
to happen
|
|
|
|
0:53:56.950,0:53:57.469
|
|
and
|
|
|
|
0:53:57.469,0:54:01.449
|
|
you know so you sort of statistically
|
|
guess that you fix that you know I was getting
|
|
|
|
0:54:01.449,0:54:03.510
|
|
this bug once every three days
|
|
|
|
0:54:03.510,0:54:06.099
|
|
and now it's gone for two weeks without happening
|
|
|
|
0:54:06.099,0:54:07.239
|
|
did you fix that?
|
|
|
|
0:54:07.239,0:54:08.969
|
|
or if you've been lucky
|
|
|
|
0:54:08.969,0:54:10.459
|
|
and and it's
|
|
|
|
0:54:10.459,0:54:14.349
|
|
that coupled with the fact that you're
|
|
dealing with hardware
|
|
|
|
0:54:14.349,0:54:18.049
|
|
and hardware rarely works the way it's documented
|
|
to work
|
|
|
|
0:54:18.049,0:54:21.770
|
|
and so you know they're doing everything that
|
|
it says you're supposed to do
|
|
|
|
0:54:21.770,0:54:26.260
|
|
it still doesn't work because you didn't set
|
|
the fiddle bit over on that other place over
|
|
|
|
0:54:26.260,0:54:26.660
|
|
there
|
|
|
|
0:54:26.660,0:54:30.479
|
|
that's not documented anywhere but if it's
|
|
not said it doesn't work
|
|
|
|
0:54:30.479,0:54:33.769
|
|
occasionally
|
|
|
|
0:54:33.769,0:54:36.110
|
|
so this is another reason that you really want
|
|
of avoid
|
|
|
|
0:54:36.110,0:54:40.459
|
|
dealing with this part of the system if
|
|
you can possibly help
|
|
|
|
0:54:40.459,0:54:44.369
|
|
okay but lets go through and and look at some
|
|
of the properties here starting up at
|
|
|
|
0:54:44.369,0:54:45.789
|
|
the user process
|
|
|
|
0:54:45.789,0:54:47.980
|
|
we're running with
|
|
|
|
0:54:47.980,0:54:51.449
|
|
preemptive scheduling
|
|
|
|
0:54:51.449,0:54:53.409
|
|
now there's several caveats here
|
|
|
|
0:54:53.409,0:54:55.239
|
|
preemptive scheduling is the default
|
|
|
|
0:54:55.239,0:54:56.970
|
|
so called shared scheduler
|
|
|
|
0:54:56.970,0:55:01.360
|
|
that is what you normally use there are other
|
|
schedulers like the real time scheduler
|
|
|
|
0:55:01.360,0:55:02.869
|
|
where what I'm saying isn't that true
|
|
|
|
0:55:02.869,0:55:05.709
|
|
we'll talk about some of the schedulers was
|
|
later
|
|
|
|
0:55:05.709,0:55:09.930
|
|
but the usual scheduler that you're running
|
|
on under UNIX is a shared scheduler
|
|
|
|
0:55:09.930,0:55:13.229
|
|
and under the shared scheduler user applications
|
|
|
|
0:55:13.229,0:55:15.159
|
|
run with pre emptive scheduling
|
|
|
|
0:55:15.159,0:55:17.449
|
|
and pre emptive scheduling means that
|
|
|
|
0:55:17.449,0:55:20.019
|
|
you run at the whim of the system
|
|
|
|
0:55:20.019,0:55:21.420
|
|
if it wants you to run
|
|
|
|
0:55:21.420,0:55:22.140
|
|
you run
|
|
|
|
0:55:22.140,0:55:25.490
|
|
once you to start running you have no guarantee
|
|
of how long you're going to run
|
|
|
|
0:55:25.490,0:55:29.370
|
|
it might like to run for three instructions
|
|
and then decide it doesn't like you many more
|
|
|
|
0:55:29.370,0:55:31.150
|
|
it wants to run something else
|
|
|
|
0:55:31.150,0:55:35.920
|
|
or you might get to run for several seconds
|
|
and in a row with the with no intervening
|
|
|
|
0:55:35.920,0:55:37.469
|
|
things interrupting you
|
|
|
|
0:55:37.469,0:55:39.719
|
|
you just don't know
|
|
|
|
0:55:39.719,0:55:40.969
|
|
and
|
|
|
|
0:55:40.969,0:55:42.839
|
|
really all you know is
|
|
|
|
0:55:42.839,0:55:43.569
|
|
that
|
|
|
|
0:55:43.569,0:55:48.239
|
|
they claim that they're using statistics
|
|
and that and that the statistics are fair
|
|
|
|
0:55:48.239,0:55:55.059
|
|
and so on average you're going to get a reasonable
|
|
amount of time but thats
|
|
|
|
0:55:55.059,0:55:57.129
|
|
up to the system you don't control that
|
|
|
|
0:55:57.129,0:55:58.439
|
|
the real point here
|
|
|
|
0:55:58.439,0:56:01.940
|
|
is that you don't have any way of creating
|
|
a critical section
|
|
|
|
0:56:01.940,0:56:04.950
|
|
you can't say okay I don't want to be interrupted
|
|
|
|
0:56:04.950,0:56:07.429
|
|
during this particular sequence of things
|
|
|
|
0:56:07.429,0:56:09.809
|
|
so you have to program
|
|
|
|
0:56:09.809,0:56:13.469
|
|
assuming that you may be interrupted at any
|
|
point
|
|
|
|
0:56:13.469,0:56:14.979
|
|
okay
|
|
|
|
0:56:14.979,0:56:18.909
|
|
the next thing is that when you're running
|
|
in a user process
|
|
|
|
0:56:18.909,0:56:20.719
|
|
you are running in
|
|
|
|
0:56:20.719,0:56:24.150
|
|
with the processor in what's called unprivileged
|
|
mode
|
|
|
|
0:56:24.150,0:56:28.109
|
|
one of the requirements for running any kind
|
|
of a UNIX system
|
|
|
|
0:56:28.109,0:56:31.759
|
|
is that you have to have a processor that
|
|
support privileged and unprivileged
|
|
|
|
0:56:31.759,0:56:33.709
|
|
two different modes of operation
|
|
|
|
0:56:33.709,0:56:37.049
|
|
in privileged mode which is what the kernel
|
|
runs in
|
|
|
|
0:56:37.049,0:56:38.950
|
|
the entire repertoire
|
|
|
|
0:56:38.950,0:56:40.869
|
|
of the hardware is available
|
|
|
|
0:56:40.869,0:56:45.339
|
|
by this I mean you can set all the registers
|
|
you can fiddle with the memory management
|
|
|
|
0:56:45.339,0:56:47.460
|
|
unit you can initiate I/O
|
|
|
|
0:56:47.460,0:56:50.519
|
|
you can access any memory anywhere
|
|
|
|
0:56:50.519,0:56:51.919
|
|
etc
|
|
|
|
0:56:51.919,0:56:56.540
|
|
when you're running in unprivileged
|
|
mode which is what user processes run in and
|
|
|
|
0:56:56.540,0:57:00.709
|
|
this a large subset of the instructions which
|
|
you cannot execute
|
|
|
|
0:57:00.709,0:57:03.480
|
|
you cannot initiate I/O on
|
|
|
|
0:57:03.480,0:57:04.209
|
|
devices
|
|
|
|
0:57:04.209,0:57:06.770
|
|
you cannot change the memory mapping
|
|
|
|
0:57:06.770,0:57:10.209
|
|
you cannot access memory that's not part of
|
|
your address space
|
|
|
|
0:57:10.209,0:57:13.299
|
|
you cannot execute certain instructions
|
|
like halt
|
|
|
|
0:57:13.299,0:57:15.589
|
|
and
|
|
|
|
0:57:15.589,0:57:19.039
|
|
so in general you are protected
|
|
|
|
0:57:19.039,0:57:21.789
|
|
from manipulating anything that's outside of your
|
|
address space
|
|
|
|
0:57:21.789,0:57:23.759
|
|
this of course is desirable because
|
|
|
|
0:57:23.759,0:57:27.059
|
|
when you're running in this unprivileged
|
|
mode
|
|
|
|
0:57:27.059,0:57:28.300
|
|
you're protected
|
|
|
|
0:57:28.300,0:57:31.910
|
|
from other processes manipulating you
|
|
and they're protected from you manipulating
|
|
|
|
0:57:31.910,0:57:33.079
|
|
them
|
|
|
|
0:57:33.079,0:57:36.430
|
|
for those of you that have had that misfortune
|
|
to have to use
|
|
|
|
0:57:36.430,0:57:39.339
|
|
early versions of windows up to about ninety
|
|
eight
|
|
|
|
0:57:39.339,0:57:42.470
|
|
they always ran with the processor
|
|
running in privileged mode
|
|
|
|
0:57:42.470,0:57:44.009
|
|
even in applications
|
|
|
|
0:57:44.009,0:57:46.459
|
|
and so either maliciously or accidentally
|
|
|
|
0:57:46.459,0:57:50.000
|
|
you could stop on other people address space
|
|
or you could stop on the kernel
|
|
|
|
0:57:50.000,0:57:53.020
|
|
and a lot of the blue screen of death was
|
|
people just
|
|
|
|
0:57:53.020,0:57:56.319
|
|
following wild pointers and trashing different
|
|
parts of the system
|
|
|
|
0:57:56.319,0:57:58.819
|
|
taking everything down
|
|
|
|
0:57:58.819,0:58:00.020
|
|
it also makes it
|
|
|
|
0:58:00.020,0:58:02.320
|
|
far easier to
|
|
|
|
0:58:02.320,0:58:05.459
|
|
implement things like viruses and worms and
|
|
other things because
|
|
|
|
0:58:05.459,0:58:09.619
|
|
a user application can we rewrite the boot
|
|
block on the disk they can just the write down
|
|
|
|
0:58:09.619,0:58:13.109
|
|
and manipulate the registers that allow them
|
|
to do whatever they want
|
|
|
|
0:58:13.109,0:58:16.730
|
|
whereas when you're running in unprivileged
|
|
mode you can't write those kinds of
|
|
|
|
0:58:16.730,0:58:20.179
|
|
of things
|
|
|
|
0:58:20.179,0:58:24.119
|
|
so modern versions of Windows anything from about
|
|
2000 on
|
|
|
|
0:58:24.119,0:58:26.630
|
|
now run with privileged and unprivileged mode
|
|
|
|
0:58:26.630,0:58:28.649
|
|
but UNIX has always required that
|
|
|
|
0:58:28.649,0:58:30.219
|
|
and so when you're running an
|
|
|
|
0:58:30.219,0:58:31.319
|
|
user process
|
|
|
|
0:58:31.319,0:58:33.389
|
|
you cannot block I mean
|
|
|
|
0:58:33.389,0:58:37.969
|
|
you cannot execute the instructions which
|
|
cause a context switching to occur
|
|
|
|
0:58:37.969,0:58:40.349
|
|
you can't pick what's going to run next
|
|
|
|
0:58:40.349,0:58:43.140
|
|
you can't make that thing run next all you can
|
|
do
|
|
|
|
0:58:43.140,0:58:45.189
|
|
is go to the operating system and say
|
|
|
|
0:58:45.189,0:58:49.269
|
|
hey I've got nothing to do. pick somebody else
|
|
to run
|
|
|
|
0:58:49.269,0:58:53.449
|
|
and the operating system is the think they can
|
|
then execute the instructions which cause
|
|
|
|
0:58:53.449,0:58:57.609
|
|
a different process to be loaded
|
|
|
|
0:58:57.609,0:58:59.049
|
|
and run
|
|
|
|
0:58:59.049,0:59:03.400
|
|
alright.finally while you're in a user application you're
|
|
running on a user stack
|
|
|
|
0:59:03.400,0:59:06.410
|
|
that's part of the user's address space
|
|
|
|
0:59:06.410,0:59:07.889
|
|
so
|
|
|
|
0:59:07.889,0:59:10.819
|
|
part of creating a process gives you a runtime
|
|
stack
|
|
|
|
0:59:10.819,0:59:14.369
|
|
as part of a virtual address space and so it
|
|
can be
|
|
|
|
0:59:14.369,0:59:18.199
|
|
more or less up to the limits of the hardware
|
|
as big as you want it to be
|
|
|
|
0:59:18.199,0:59:19.949
|
|
so if you are running on thirty two-bit processor
|
|
|
|
0:59:19.949,0:59:22.819
|
|
you're stack can get the 2 gigabytes
|
|
|
|
0:59:22.819,0:59:23.319
|
|
and
|
|
|
|
0:59:23.319,0:59:26.839
|
|
the what this means is that anytime you
|
|
allocate local variables
|
|
|
|
0:59:26.839,0:59:28.529
|
|
you don't have to worry about Oh
|
|
|
|
0:59:28.529,0:59:30.609
|
|
is that gonna overrun my stack?
|
|
|
|
0:59:30.609,0:59:31.610
|
|
so if you need
|
|
|
|
0:59:31.610,0:59:35.519
|
|
a hundred thousand double precision floating
|
|
point numbers
|
|
|
|
0:59:35.519,0:59:37.189
|
|
you can just as a local variable allocate
|
|
|
|
0:59:37.189,0:59:40.269
|
|
an array of size a hundred-thousand type
|
|
double
|
|
|
|
0:59:40.269,0:59:44.029
|
|
and it just decrements your stack pointer by
|
|
hundred hundred thousand bytes
|
|
|
|
0:59:44.029,0:59:45.009
|
|
away you go
|
|
|
|
0:59:45.009,0:59:47.299
|
|
it's just virtual address space
|
|
|
|
0:59:47.299,0:59:49.020
|
|
as you'll see when we get into the kernel
|
|
|
|
0:59:49.020,0:59:50.210
|
|
that ceases to be the case
|