Subject: + documentation-sysctl-vmtxt-clarify-vfs_cache_pressure-description.patch added to -mm tree
To: dvlasenk@xxxxxxxxxx,viro@xxxxxxxxxxxxxxxxxx
From: akpm@xxxxxxxxxxxxxxxxxxxx
Date: Mon, 02 Jun 2014 16:22:53 -0700
The patch titled
Subject: Documentation/sysctl/vm.txt: clarify vfs_cache_pressure description
has been added to the -mm tree. Its filename is
documentation-sysctl-vmtxt-clarify-vfs_cache_pressure-description.patch
This patch should soon appear at
http://ozlabs.org/~akpm/mmots/broken-out/documentation-sysctl-vmtxt-clarify-vfs_cache_pressure-description.patch
and later at
http://ozlabs.org/~akpm/mmotm/broken-out/documentation-sysctl-vmtxt-clarify-vfs_cache_pressure-description.patch
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------------------------------------------------------
From: Denys Vlasenko <dvlasenk@xxxxxxxxxx>
Subject: 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@xxxxxxxxxx>
Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
Documentation/sysctl/vm.txt | 9 +++++++--
1 file changed, 7 insertions(+), 2 deletions(-)
diff -puN Documentation/sysctl/vm.txt~documentation-sysctl-vmtxt-clarify-vfs_cache_pressure-description Documentation/sysctl/vm.txt
--- a/Documentation/sysctl/vm.txt~documentation-sysctl-vmtxt-clarify-vfs_cache_pressure-description
+++ a/Documentation/sysctl/vm.txt
@@ -746,8 +746,8 @@ Changing this takes effect whenever an a
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
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:
_
Patches currently in -mm which might be from dvlasenk@xxxxxxxxxx are
documentation-sysctl-vmtxt-clarify-vfs_cache_pressure-description.patch
linux-next.patch
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