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Message-Id: <1400768449-16784-1-git-send-email-dvlasenk@redhat.com> Date: Thu, 22 May 2014 16:20:49 +0200 From: Denys Vlasenko <dvlasenk@...hat.com> To: linux-kernel@...r.kernel.org Cc: Denys Vlasenko <dvlasenk@...hat.com>, Alexander Viro <viro@...iv.linux.org.uk> Subject: [PATCH] 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. >From the 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@...hat.com> Cc: Alexander Viro <viro@...iv.linux.org.uk> --- Documentation/sysctl/vm.txt | 9 +++++++-- 1 file changed, 7 insertions(+), 2 deletions(-) diff --git a/Documentation/sysctl/vm.txt b/Documentation/sysctl/vm.txt index dd9d0e3..2449ec54 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: -- 1.8.1.4 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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