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Add report on vnode cache tuning from mckusick

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Benjamin Kaduk 2016-01-09 19:23:42 +00:00
parent 9cfe77d6fd
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Notes: svn2git 2020-12-08 03:00:23 +00:00 
svn path=/head/; revision=47974
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en_US.ISO8859-1/htdocs/news/status/report-2015-10-2015-12.xml
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@ -558,4 +558,112 @@
portions and committed.</p>
</body>
</project>
<project cat='kern'>
<title>Kernel Vnode Cache Tuning</title>
<contact>
<person>
<name>
<given>Kirk</given>
<common>McKusick</common>
</name>
<email>mckusick@mckusick.com</email>
</person>
<person>
<name>
<given>Bruce</given>
<common>Evans</common>
</name>
<email>bde@FreeBSD.org</email>
</person>
<person>
<name>
<given>Konstantin</given>
<common>Belousov</common>
</name>
<email>kib@FreeBSD.org</email>
</person>
<person>
<name>
<given>Peter</given>
<common>Holm</common>
</name>
<email>pho@FreeBSD.org</email>
</person>
<person>
<name>
<given>Mateusz</given>
<common>Guzik</common>
</name>
<email>mjg@FreeBSD.org</email>
</person>
</contact>
<links>
<url href="https://reviews.FreeBSD.org/rS292895">MFC to stable/10</url>
</links>
<body>
<p>This completed project includes changes to better manage
the vnode freelist and to streamline the allocation and freeing of
vnodes.</p>
<p>Vnode cache recycling was reworked to meet free and unused
vnodes targets. Free vnodes are rarely completely free; rather,
they are just ones that are cheap to recycle. Usually they are
for files which have been stat'd but not read; these usually have
inode and namecache data attached to them. The free vnode target
is the preferred minimum size of a sub-cache consisting mostly of
such files. The system balances the size of this sub-cache with
its complement to try to prevent either from thrashing while the
other is relatively inactive. The targets express a preference
for the best balance.</p>
<p>&quot;Above&quot; this target there are 2 further targets
(watermarks) related to the recyling of free vnodes. In the
best-operating case, the cache is exactly full, the free list has
size between vlowat and vhiwat above the free target, and
recycling from the free list and normal use maintains this state.
Sometimes the free list is below vlowat or even empty, but this
state is even better for immediate use, provided the cache is not
full. Otherwise, vnlru_proc() runs to reclaim enough vnodes
(usually non-free ones) to reach one of these states. The
watermarks are currently hard-coded as 4% and 9% of the available
space. These, and the default of 25% for wantfreevnodes, are too
large if the memory size is large. E.g., 9% of 75% of MAXVNODES
is more than 566000 vnodes to reclaim whenever vnlru_proc()
becomes active.</p>
<p>The <tt>vfs.vlru_alloc_cache_src</tt> sysctl is removed.
New code frees namecache sources as the last chance to satisfy the
highest watermark, instead of selecting source vnodes randomly.
This provides good enough behaviour to keep vn_fullpath() working
in most situations. Filesystem layouts with deep trees, where the
removed knob was required, is thus handled automatically.</p>
<p>As the kernel allocates and frees vnodes, it fully
initializes them on every allocation and fully releases them on
every free. These are not trivial costs: it starts by zeroing a
large structure, then initializes a mutex, a lock manager lock, an
rw lock, four lists, and six pointers. Looking at
<tt>vfs.vnodes_created</tt>, these operations are being done
millions of times an hour on a busy machine.</p>
<p>As a performance optimization, this code update uses the
uma_init and uma_fini routines to do these initializations and
cleanups only as the vnodes enter and leave the vnode zone. With
this change, the initializations are done <tt>kern.maxvnodes</tt>
times at system startup, and then only rarely again. The frees
are done only if the vnode zone shrinks, which never happens in
practice. For those curious about the avoided work, look at the
vnode_init() and vnode_fini() functions in sys/kern/vfs_subr.c to
see the code that has been removed from the main vnode
allocation/free path.</p>
</body>
</project>
</report>