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Add entry on OOM updates from kib

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Benjamin Kaduk 2016-01-09 21:30:57 +00:00
parent 40980769ef
commit cebdb7ee46
Notes: svn2git 2020-12-08 03:00:23 +00:00 
svn path=/head/; revision=47983
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en_US.ISO8859-1/htdocs/news/status/report-2015-10-2015-12.xml
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</task>
</help>
</project>
<project cat='kern'>
<title>Out of Memory Handler Rewrite</title>
<contact>
<person>
<name>
<given>Konstantin</given>
<common>Belousov</common>
</name>
<email>kib@FreeBSD.org</email>
</person>
</contact>
<body>
<p>The Out of Memory (OOM) code is intended to handle the
situation where the system needs free memory to make progress,
while no memory can be reused. Most often, the situation is that
to free memory, the system needs more free memory. Consider a
case where the system needs to page-out dirty pages, but needs to
allocate structures to track the writes. OOM &quot;solves&quot;
the problem by killing some selection of user processes. In other
words, it trades away system deadlock by suffering a partial loss
of user data. The assumption is that it is better to kill a
process and recover data in other processes, than lose
everything.</p>
<p>Free memory in the &os; Virtual Memory (VM) system appears
from two sources. One is the voluntary reclamation of pages used
by a process, for example unmapping private anonymous regions, or
the last unlink of an otherwise unreferenced file with cached
pages. Another source is the pagedaemon, which forcefully frees
pages which carry data, of course, after the data is moved to some
other storage, like swap or file blocks. OOM is triggered when
the pagedaemon definitely cannot free memory to satisfy the
requests.</p>
<p>The old criteria to trigger OOM action was a combination of
low free swap space and a low count of free pages (the later is
expressed precisely with the paging targets constants, but this is
not relevant to the discussion). That test is mostly incorrect,
e.g., a low free page state might be caused by a greedy consumer
allocating all pages freed by the page daemon in the current pass,
but this does not preclude the page daemon from producing more
pages. Also, since page-outs are asynchronous, the previous page
daemon pass might not immmediately produce any free pages, but
they would appear some short time later.</p>
<p>More seriously, low swap space does not necessarily indicate
that we are in trouble: lots of pages may not require swap
allocations to freed, e.g., clean pages or pages backed by files.
The last notion is serious, since swap-less systems were
considered as having full swap.</p>
<p>Instead of trying to deduce the deadlock from looking at
the current VM state, the new OOM handler tracks the history of
page daemon passes. Only if several consequtive passes failed to
meet the paging target is an OOM kill considered neccessary. The
count of consequent failed passes was selected empirically, by
testing on small (32M) and large (512G) machines. Auto-tuning of
the counter is possible, but requires some more architectural
changes to the I/O subsystem.</p>
<p>Another issue was identified with the algorithm which
selects a victim process for OOM kill. It compared the counts of
pages mapping entries (PTEs) installed into the machine paging
structures. For different reasons, machine-dependent VM code
(pmap) may remove the pte for a memory-resident page. Under some
circumstances, related to other measures to prevent low memory
deadlock, very large processes which consume all system memory,
could have few or no ptes, and the old OOM selector ignored the
process which caused the deadlock, killing unrelated
processes.</p>
<p>A new function vm_pageout_oom_pagecount() was written which
applies a reasonable heuristic to estimate the number of pages
which would be freed by killing the given process. This
eliminates the effect of selecting small unrelated processes for
OOM kill.</p>
<p>The rewrite was committed to HEAD in r290917 and r290920.</p>
</body>
<sponsor>The FreeBSD Foundation</sponsor>
</project>
</report>