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Documentation/sysctl/vm.txt: clarify vfs_cache_pressure description

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Existing description is worded in a way which almost encourages setting of
vfs_cache_pressure above 100, possibly way above it.

Users are left in a dark what this numeric value is - an int?  a
percentage?  what the scale is?

As a result, we are getting reports about noticeable performance
degradation from users who have set vfs_cache_pressure to ridiculously
high values - because they thought there is no downside to it.

Via code inspection it's obvious that this value is treated as a
percentage.  This patch changes text to reflect this fact, and adds a
cautionary paragraph advising against setting vfs_cache_pressure sky high.

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Denys Vlasenko 2014-06-04 16:11:03 -07:00 committed by Linus Torvalds
parent 3ba08129e3
commit 4a0da71b96
1 changed files with 7 additions and 2 deletions
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9
Documentation/sysctl/vm.txt
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@ -746,8 +746,8 @@ Changing this takes effect whenever an application requests memory.
vfs_cache_pressure vfs_cache_pressure
------------------ ------------------
Controls the tendency of the kernel to reclaim the memory which is used for This percentage value controls the tendency of the kernel to reclaim
caching of directory and inode objects. the memory which is used for caching of directory and inode objects.
At the default value of vfs_cache_pressure=100 the kernel will attempt to At the default value of vfs_cache_pressure=100 the kernel will attempt to
reclaim dentries and inodes at a "fair" rate with respect to pagecache and reclaim dentries and inodes at a "fair" rate with respect to pagecache and
@ -757,6 +757,11 @@ never reclaim dentries and inodes due to memory pressure and this can easily
lead to out-of-memory conditions. Increasing vfs_cache_pressure beyond 100 lead to out-of-memory conditions. Increasing vfs_cache_pressure beyond 100
causes the kernel to prefer to reclaim dentries and inodes. causes the kernel to prefer to reclaim dentries and inodes.
Increasing vfs_cache_pressure significantly beyond 100 may have negative
performance impact. Reclaim code needs to take various locks to find freeable
directory and inode objects. With vfs_cache_pressure=1000, it will look for
ten times more freeable objects than there are.
============================================================== ==============================================================
zone_reclaim_mode: zone_reclaim_mode: