Add section on VM system tuning

svn path=/head/; revision=4252

handbook/vm.sgml

@@ -1,4 +1,4 @@
-<!-- $Id: vm.sgml,v 1.1 1999-02-06 20:20:17 dillon Exp $ -->
+<!-- $Id: vm.sgml,v 1.2 1999-02-08 20:21:36 dillon Exp $ -->
 <!-- The FreeBSD Documentation Project -->
 
 <sect><heading>The FreeBSD VM System<label id="vm"></heading>
@@ -160,6 +160,64 @@
 	utilization broken down by zone.
 	<p>
 
+<sect1><heading>Tuning the FreeBSD VM system</heading>
+	<p>
+	A concerted effort has been made to make the FreeBSD kernel dynamically
+	tune itself.  Typically you do not need to mess with anything beyond
+	the 'maxusers' and 'NMBCLUSTERS' kernel config options.  That is,
+	kernel compilation options specified in ( typically )
+	/usr/src/sys/i386/conf/XXX.  A description of all available kernel
+	configuration options can be found in /usr/src/sys/i386/conf/LINT.
+	<p>
+	In a large system configuration you may wish to increase 'maxusers'.
+	Values typically range from 10 to 256.
+	<p>
+	If your system is going to use the network heavily, you may want
+	to increase NMBCLUSTERS.  Typical values range from 1024 to 4096.
+	Each cluster represents around 16KBytes so do not overdo it.
+	<p>
+	The NBUF parameter is also traditionally used to scale the system.
+	This parameter determines the amount of KVA the system can use to
+	map filesystem buffers for I/O.  Note that this parameter has nothing
+	whatsoever to do with the unified buffer cache!   This parameter
+	is dynamically tuned in -3.x and later kernels and should generally not
+	be adjusted manually.  We recommend that you NOT try to specify an
+	NBUF parameter.  Let the system pick it.  Too small a value can result
+	in extremely inefficient filesystem operation while too large a value
+	can starve the page queues by causing too many pages to become wired
+	down.
+	<p>
+	By default, FreeBSD kernels are not optimized.  You can set debugging
+	and optimization flags with the 'makeoptions' directive in the kernel
+	configuration.  Note that you should not use -g unless you can 
+	accomodate the large ( typically 7 MB+ ) kernels that result.
+	<p><tt>makeoptions    DEBUG="-g"</tt>
+	<p><tt>makeoptions     COPTFLAGS="-O2 -pipe"</tt>
+	<p>
+	Sysctl provides a way to tune kernel parameters at run-time.  You
+	typically do not need to mess with any of the sysctl variables,
+	especially the VM related ones.
+	<p>
+	Run time VM and system tuning is relatively straightforward.  First,
+	use softupdates on your UFS/FFS filesystems whenever possible.  
+	The /usr/src/contrib/sys/softupdates/README file contains instructions
+	( and restrictions ) on how to configure it up. 
+	<p>
+	Second, configure 
+	sufficient swap.  You should have a swap partition configured on each
+	physical disk, up to four, even on your 'work' disks.  You should have
+	at least 2x the swap space as you have main memory, and possibly even
+	more if you do not have a lot of memory.  You should also size your
+	swap partition based on the maximum memory configuration you ever
+	intend to put on the machine so you do not have to repartition your
+	disks later on.  If you want to be able to accomodate a crash dump,
+	your first swap partition must be at least as large as main memory
+	and /var/crash must have sufficient free space to hold the dump.
+	<p>
+	NFS-based swap is perfectly acceptable on -4.x or later systems, but
+	you must be aware that the NFS server will take the brunt of the 
+	paging load.
+
 <em>Contributed by &a.dillon;.<newline>
   6 Feb 1999.</em>