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authorDenys Vlasenko <dvlasenk@redhat.com>2014-06-04 16:11:03 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2014-06-04 16:54:13 -0700
commit4a0da71b96b9d4080c0820e9e7d02470ebe62dc6 (patch)
treed5fd8eb6b6f5d8877a5f84ddffa7f3e6a33e8284 /Documentation
parent3ba08129e38437561df44c36b7ea9081185d5333 (diff)
downloadop-kernel-dev-4a0da71b96b9d4080c0820e9e7d02470ebe62dc6.zip
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Documentation/sysctl/vm.txt: clarify vfs_cache_pressure description
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>
Diffstat (limited to 'Documentation')
-rw-r--r--Documentation/sysctl/vm.txt9
1 files changed, 7 insertions, 2 deletions
diff --git a/Documentation/sysctl/vm.txt b/Documentation/sysctl/vm.txt
index 5b6da0f..bd4b34c 100644
--- a/Documentation/sysctl/vm.txt
+++ b/Documentation/sysctl/vm.txt
@@ -746,8 +746,8 @@ Changing this takes effect whenever an application requests memory.
vfs_cache_pressure
------------------
-Controls the tendency of the kernel to reclaim the memory which is used for
-caching of directory and inode objects.
+This percentage value controls the tendency of the kernel to reclaim
+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
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
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:
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