svn commit: r47974 - head/en_US.ISO8859-1/htdocs/news/status
Benjamin Kaduk <[email protected]>
| Newsgroups | gmane.os.freebsd.devel.cvs.doc |
|---|---|
| Message-ID | <[email protected]> |
Author: bjk
Date: Sat Jan 9 19:23:42 2016
New Revision: 47974
URL: https://svnweb.freebsd.org/changeset/doc/47974
Log:
Add report on vnode cache tuning from mckusick
Modified:
head/en_US.ISO8859-1/htdocs/news/status/report-2015-10-2015-12.xml
Modified: head/en_US.ISO8859-1/htdocs/news/status/report-2015-10-2015-12.xml
==============================================================================
--- head/en_US.ISO8859-1/htdocs/news/status/report-2015-10-2015-12.xml Sat Jan 9 19:08:52 2016 (r47973)
+++ head/en_US.ISO8859-1/htdocs/news/status/report-2015-10-2015-12.xml Sat Jan 9 19:23:42 2016 (r47974)
@@ -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>[email protected]</email>
+ </person>
+
+ <person>
+ <name>
+ <given>Bruce</given>
+ <common>Evans</common>
+ </name>
+ <email>[email protected]</email>
+ </person>
+
+ <person>
+ <name>
+ <given>Konstantin</given>
+ <common>Belousov</common>
+ </name>
+ <email>[email protected]</email>
+ </person>
+
+ <person>
+ <name>
+ <given>Peter</given>
+ <common>Holm</common>
+ </name>
+ <email>[email protected]</email>
+ </person>
+
+ <person>
+ <name>
+ <given>Mateusz</given>
+ <common>Guzik</common>
+ </name>
+ <email>[email protected]</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>"Above" 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>
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