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Update FAQ entries 10.21 and 10.22

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PR:		242062
Submitted by:	pauamma@gundo.com
Reviewed by:	markj, debdrup
Differential Revision:	https://reviews.freebsd.org/D24486
This commit is contained in:
Allan Jude 2020-04-28 02:28:48 +00:00
parent e5ac16fbf5
commit 6e0b998f99
Notes: svn2git 2020-12-08 03:00:23 +00:00 
svn path=/head/; revision=54079
1 changed files with 28 additions and 37 deletions
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en_US.ISO8859-1/books/faq/book.xml
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@ -4003,7 +4003,7 @@ options SYSVMSG # enable for messaging</programlisting>
<command>top</command> mean?</para> <command>top</command> mean?</para>
</question> </question>
<!-- Provided by John Dyson via Usenet --> <!-- Provided by John Dyson via Usenet, and later amended -->
<answer> <answer>
<itemizedlist> <itemizedlist>
<listitem> <listitem>
@ -4017,24 +4017,15 @@ options SYSVMSG # enable for messaging</programlisting>
</listitem> </listitem>
<listitem> <listitem>
<para><literal>Cache</literal>: (most often) pages that <para><literal>Laundry</literal>: pages recently
have percolated from inactive to a status where they statistically unused but known to be dirty, that is,
maintain their data, but can often be immediately whose contents needs to be paged out before they can
reused (either with their old association, or reused be reused.</para>
with a new association). There can be certain
immediate transitions from <literal>active</literal>
to <literal>cache</literal> state if the page is known
to be clean (unmodified), but that transition is a
matter of policy, depending upon the algorithm choice
of the VM system maintainer.</para>
</listitem> </listitem>
<listitem> <listitem>
<para><literal>Free</literal>: pages without data <para><literal>Free</literal>: pages without data
content, and can be immediately used in certain content, which can be immediately reused.</para>
circumstances where cache pages might be ineligible.
Free pages can be reused at interrupt or process
state.</para>
</listitem> </listitem>
<listitem> <listitem>
@ -4045,21 +4036,19 @@ options SYSVMSG # enable for messaging</programlisting>
</itemizedlist> </itemizedlist>
<para>Pages are most often written to disk (sort of a VM <para>Pages are most often written to disk (sort of a VM
sync) when they are in the inactive state, but active sync) when they are in the laundry state, but active or
pages can also be synced. This depends upon the CPU inactive pages can also be synced. This depends upon the
tracking of the modified bit being available, and in CPU tracking of the modified bit being available, and in
certain situations there can be an advantage for a block certain situations there can be an advantage for a block
of VM pages to be synced, whether they are active or of VM pages to be synced, regardless of the queue they
inactive. In most common cases, it is best to think of belong to. In most common cases, it is best to think of
the inactive queue to be a queue of relatively unused the laundry queue as a queue of relatively unused
pages that might or might not be in the process of being pages that might or might not be in the process of being
written to disk. Cached pages are already synced, not written to disk. The inactive queue contains a mix of
mapped, but available for immediate process use with their clean and dirty pages; clean pages near the head of the
old association or with a new association. Free pages are queue are reclaimed immediately to alleviate a free page
available at interrupt level, but cached or free pages can shortage, and dirty pages are moved to the laundry queue
be used at process state for reuse. Cache pages are not for deferred processing.</para>
adequately locked to be available at interrupt
level.</para>
<para>There are some other flags (e.g., busy flag or busy <para>There are some other flags (e.g., busy flag or busy
count) that might modify some of the described count) that might modify some of the described
@ -4072,17 +4061,19 @@ options SYSVMSG # enable for messaging</programlisting>
<para>How much free memory is available?</para> <para>How much free memory is available?</para>
</question> </question>
<!-- Provided by John Dyson via Usenet --> <!-- Provided by John Dyson via Usenet, and later amended -->
<answer> <answer>
<para>There are a couple of kinds of <quote>free <para>There are a couple of kinds of <quote>free
memory</quote>. One kind is the amount of memory memory</quote>. The most common is the amount of memory
immediately available without paging anything else out. immediately available without reclaiming memory already
That is approximately the size of cache queue + size of in use. That is the size of the free pages queue plus
free queue (with a derating factor, depending upon system some other reserved pages. This amount is exported by the
tuning). Another kind of <quote>free memory</quote> is <literal>vm.stats.vm.v_free_count</literal>
the total amount of <acronym>VM</acronym> space. That can &man.sysctl.8;, shown, for instance, by &man.top.1;.
be complex, but is dependent upon the amount of swap space Another kind of <quote>free memory</quote> is
and memory. Other kinds of <quote>free memory</quote> the total amount of virtual memory available to userland
processes, which depends on the sum of swap space and
usable memory. Other kinds of <quote>free memory</quote>
descriptions are also possible, but it is relatively descriptions are also possible, but it is relatively
useless to define these, but rather it is important to useless to define these, but rather it is important to
make sure that the paging rate is kept low, and to avoid make sure that the paging rate is kept low, and to avoid