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Add a new FAQ entry about the memory states displayed by top.

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(answer from a John Dyson newgroup post)
PR:		docs/38117
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Murray Stokely 2003-05-05 04:08:22 +00:00
parent 8c4d32e232
commit c2f098f25f
Notes: svn2git 2020-12-08 03:00:23 +00:00 
svn path=/head/; revision=16797
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@ -7276,6 +7276,67 @@ define(`confDELIVERY_MODE',`deferred')dnl</programlisting>
accessing tens of thousands of tiny files.</para>
</answer>
</qandaentry>
<qandaentry>
<question id="top-memory-states">
<para>What do the various memory states displayed by
<command>top</command> mean?</para>
</question>
<answer>
<itemizedlist>
<listitem><para><literal>Active</literal>: pages recently
statistically used.</para></listitem>
<listitem><para><literal>Inactive</literal>: pages
recently statistically unused.</para></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 transition from active to 'cache' 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><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></listitem>
<listitem><para><literal>Wired</literal>: pages that are
fixed into memory, usually for kernel purposes, but also
sometimes for special use in
processes.</para></listitem>
</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 also (but requires the
availability of certain CPU features.) 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
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 aren't
adequately locked to be available at interrupt
level.</para>
<para>There are some other flags (e.g. Busy flag or busy
count) that might modify some of the rules that I
described.</para>
</answer>
</qandaentry>
</qandaset>
</chapter>