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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>
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@ -746,8 +746,8 @@ Changing this takes effect whenever an application requests memory.
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vfs_cache_pressure
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------------------
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Controls the tendency of the kernel to reclaim the memory which is used for
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caching of directory and inode objects.
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This percentage value controls the tendency of the kernel to reclaim
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the memory which is used for caching of directory and inode objects.
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At the default value of vfs_cache_pressure=100 the kernel will attempt to
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reclaim dentries and inodes at a "fair" rate with respect to pagecache and
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@ -757,6 +757,11 @@ never reclaim dentries and inodes due to memory pressure and this can easily
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lead to out-of-memory conditions. Increasing vfs_cache_pressure beyond 100
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causes the kernel to prefer to reclaim dentries and inodes.
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Increasing vfs_cache_pressure significantly beyond 100 may have negative
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performance impact. Reclaim code needs to take various locks to find freeable
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directory and inode objects. With vfs_cache_pressure=1000, it will look for
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ten times more freeable objects than there are.
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==============================================================
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zone_reclaim_mode:
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