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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>
</question>
<!-- Provided by John Dyson via Usenet -->
<!-- Provided by John Dyson via Usenet, and later amended -->
<answer>
<itemizedlist>
<listitem>
@ -4017,24 +4017,15 @@ options SYSVMSG # enable for messaging</programlisting>
</listitem>
<listitem>
<para><literal>Cache</literal>: (most often) pages that
have percolated from inactive to a status where they
maintain their data, but can often be immediately
reused (either with their old association, or reused
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>
<para><literal>Laundry</literal>: pages recently
statistically unused but known to be dirty, that is,
whose contents needs to be paged out before they can
be reused.</para>
</listitem>
<listitem>
<para><literal>Free</literal>: pages without data
content, and can be immediately used in certain
circumstances where cache pages might be ineligible.
Free pages can be reused at interrupt or process
state.</para>
content, which can be immediately reused.</para>
</listitem>
<listitem>
@ -4045,21 +4036,19 @@ options SYSVMSG # enable for messaging</programlisting>
</itemizedlist>
<para>Pages are most often written to disk (sort of a VM
sync) when they are in the inactive state, but active
pages can also be synced. This depends upon the CPU
tracking of the modified bit being available, and in
sync) when they are in the laundry state, but active or
inactive pages can also be synced. This depends upon the
CPU tracking of the modified bit being available, and in
certain situations there can be an advantage for a block
of VM pages to be synced, whether they are active or
inactive. In most common cases, it is best to think of
the inactive queue to be a queue of relatively unused
of VM pages to be synced, regardless of the queue they
belong to. In most common cases, it is best to think of
the laundry queue as a queue of relatively unused
pages that might or might not be in the process of being
written to disk. Cached pages are already synced, not
mapped, but available for immediate process use with their
old association or with a new association. Free pages are
available at interrupt level, but cached or free pages can
be used at process state for reuse. Cache pages are not
adequately locked to be available at interrupt
level.</para>
written to disk. The inactive queue contains a mix of
clean and dirty pages; clean pages near the head of the
queue are reclaimed immediately to alleviate a free page
shortage, and dirty pages are moved to the laundry queue
for deferred processing.</para>
<para>There are some other flags (e.g., busy flag or busy
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>
</question>
<!-- Provided by John Dyson via Usenet -->
<!-- Provided by John Dyson via Usenet, and later amended -->
<answer>
<para>There are a couple of kinds of <quote>free
memory</quote>. One kind is the amount of memory
immediately available without paging anything else out.
That is approximately the size of cache queue + size of
free queue (with a derating factor, depending upon system
tuning). Another kind of <quote>free memory</quote> is
the total amount of <acronym>VM</acronym> space. That can
be complex, but is dependent upon the amount of swap space
and memory. Other kinds of <quote>free memory</quote>
memory</quote>. The most common is the amount of memory
immediately available without reclaiming memory already
in use. That is the size of the free pages queue plus
some other reserved pages. This amount is exported by the
<literal>vm.stats.vm.v_free_count</literal>
&man.sysctl.8;, shown, for instance, by &man.top.1;.
Another kind of <quote>free memory</quote> is
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
useless to define these, but rather it is important to
make sure that the paging rate is kept low, and to avoid