swap: improve swap I/O rate - V2

From: Ehrhardt Christian <ehrhardt@linux.vnet.ibm.com>

From: Christian Ehrhardt <ehrhardt@linux.vnet.ibm.com>

* Update in V3 *
- Added another reviewed by
- should be ready for upstream inclusion now

* Update in V2 *
- Adapted the documentation patch according to feedback of Minchan Kim
- Added the Acks I got to V1 so far

In an memory overcommitment scneario with KVM I ran into a lot of waits for
swap. While checking the I/O done on the swap disks I found almost all I/Os
to be done as single page 4k request. Despite the fact that swap in is a
batch of 1<<page-cluster pages as swap readahead and swap out is a list of
pages written in shrink_page_list.

[1/2 swap in improvment]
The read patch shows improvements of up to 50% swap throughput, much happier
guest systems and even when running with comparable throughput a lot I/O per
seconds saved leaving resources in the SAN for other consumers.

[2/2 documentation]
While doing so I also realized that the documentation for
proc/sys/vm/page-cluster is no more matching the code

Kind regards,
Christian Ehrhardt


Christian Ehrhardt (2):
  swap: allow swap readahead to be merged
  documentation: update how page-cluster affects swap I/O

 Documentation/sysctl/vm.txt |   12 ++++++++++--
 mm/swap_state.c             |    5 +++++
 2 files changed, 15 insertions(+), 2 deletions(-)

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