X-Git-Url: http://git.cascardo.info/?a=blobdiff_plain;f=Documentation%2Fsysctl%2Fvm.txt;h=bd4b34c0373878afc200173b4955724ab9899fa2;hb=9d2cd01b15d0782adb81e40094b67904d77b03df;hp=dd9d0e33b44391200c2004584e79d66baf527991;hpb=40e9963e622cf28ecef258e3dddb04457b65681c;p=cascardo%2Flinux.git

diff --git a/Documentation/sysctl/vm.txt b/Documentation/sysctl/vm.txt
index dd9d0e33b443..bd4b34c03738 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:
@@ -772,16 +777,17 @@ This is value ORed together of
 2	= Zone reclaim writes dirty pages out
 4	= Zone reclaim swaps pages
 
-zone_reclaim_mode is set during bootup to 1 if it is determined that pages
-from remote zones will cause a measurable performance reduction. The
-page allocator will then reclaim easily reusable pages (those page
-cache pages that are currently not used) before allocating off node pages.
-
-It may be beneficial to switch off zone reclaim if the system is
-used for a file server and all of memory should be used for caching files
-from disk. In that case the caching effect is more important than
+zone_reclaim_mode is disabled by default.  For file servers or workloads
+that benefit from having their data cached, zone_reclaim_mode should be
+left disabled as the caching effect is likely to be more important than
 data locality.
 
+zone_reclaim may be enabled if it's known that the workload is partitioned
+such that each partition fits within a NUMA node and that accessing remote
+memory would cause a measurable performance reduction.  The page allocator
+will then reclaim easily reusable pages (those page cache pages that are
+currently not used) before allocating off node pages.
+
 Allowing zone reclaim to write out pages stops processes that are
 writing large amounts of data from dirtying pages on other nodes. Zone
 reclaim will write out dirty pages if a zone fills up and so effectively