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Commit df858fa8 authored by Christian Ehrhardt's avatar Christian Ehrhardt Committed by Linus Torvalds
Browse files

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: default avatarChristian Ehrhardt <ehrhardt@linux.vnet.ibm.com>
Acked-by: default avatarJens Axboe <axboe@kernel.dk>
Reviewed-by: default avatarMinchan Kim <minchan@kernel.org>
Cc: Hugh Dickins <hughd@google.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 3fb5c298

Documentation/sysctl/vm.txt

+10 −2
Original line number Diff line number Diff line
@@ -574,16 +574,24 @@ of physical RAM. See above. 


page-cluster



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

swap-intensive.



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.



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