diff options
authorChristian Ehrhardt <ehrhardt@linux.vnet.ibm.com>2012-07-31 16:41:46 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2012-07-31 18:42:39 -0700
commitdf858fa8276f85106f2f5c3cd49c1fa524058070 (patch)
parent3fb5c298b04eb6e472f8db1f0fb472749d30041c (diff)
documentation: update how page-cluster affects swap I/O
Fix of the documentation of /proc/sys/vm/page-cluster to match the behavior of the code and add some comments about what the tunable will change in that behavior. Signed-off-by: Christian Ehrhardt <ehrhardt@linux.vnet.ibm.com> Acked-by: Jens Axboe <axboe@kernel.dk> Reviewed-by: Minchan Kim <minchan@kernel.org> Cc: Hugh Dickins <hughd@google.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
1 files changed, 10 insertions, 2 deletions
diff --git a/Documentation/sysctl/vm.txt b/Documentation/sysctl/vm.txt
index 96f0ee825bed..84eb25cd69aa 100644
--- a/Documentation/sysctl/vm.txt
+++ b/Documentation/sysctl/vm.txt
@@ -574,16 +574,24 @@ of physical RAM. See above.
-page-cluster controls the number of pages which are written to swap in
-a single attempt. The swap I/O size.
+page-cluster controls the number of pages up to which consecutive pages
+are read in from swap in a single attempt. This is the swap counterpart
+to page cache readahead.
+The mentioned consecutivity is not in terms of virtual/physical addresses,
+but consecutive on swap space - that means they were swapped out together.
It is a logarithmic value - setting it to zero means "1 page", setting
it to 1 means "2 pages", setting it to 2 means "4 pages", etc.
+Zero disables swap readahead completely.
The default value is three (eight pages at a time). There may be some
small benefits in tuning this to a different value if your workload is
+Lower values mean lower latencies for initial faults, but at the same time
+extra faults and I/O delays for following faults if they would have been part of
+that consecutive pages readahead would have brought in.