path: root/Documentation
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authorChristian Ehrhardt <ehrhardt@linux.vnet.ibm.com>2012-07-31 16:41:44 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2012-07-31 18:42:39 -0700
commit3fb5c298b04eb6e472f8db1f0fb472749d30041c (patch)
treeac3db5588e569a139e417c6ddc69d3d95cb90ad9 /Documentation
parenta7d6f529fe1d96a477614eb93f40213d133029e6 (diff)
swap: allow swap readahead to be merged
Swap readahead works fine, but the I/O to disk is almost always done in page size requests, despite the fact that readahead submits 1<<page-cluster pages at a time. On older kernels the old per device plugging behavior might have captured this and merged the requests, but currently all comes down to much more I/Os than required. On a single device this might not be an issue, but as soon as a server runs on shared san resources savin I/Os not only improves swapin throughput but also provides a lower resource utilization. With a load running KVM in a lot of memory overcommitment (the hot memory is 1.5 times the host memory) swapping throughput improves significantly and the lead feels more responsive as well as achieves more throughput. In a test setup with 16 swap disks running blocktrace on one of those disks shows the improved merging: Prior: Reads Queued: 560,888, 2,243MiB Writes Queued: 226,242, 904,968KiB Read Dispatches: 544,701, 2,243MiB Write Dispatches: 159,318, 904,968KiB Reads Requeued: 0 Writes Requeued: 0 Reads Completed: 544,716, 2,243MiB Writes Completed: 159,321, 904,980KiB Read Merges: 16,187, 64,748KiB Write Merges: 61,744, 246,976KiB IO unplugs: 149,614 Timer unplugs: 2,940 With the patch: Reads Queued: 734,315, 2,937MiB Writes Queued: 300,188, 1,200MiB Read Dispatches: 214,972, 2,937MiB Write Dispatches: 215,176, 1,200MiB Reads Requeued: 0 Writes Requeued: 0 Reads Completed: 214,971, 2,937MiB Writes Completed: 215,177, 1,200MiB Read Merges: 519,343, 2,077MiB Write Merges: 73,325, 293,300KiB IO unplugs: 337,130 Timer unplugs: 11,184 I got ~10% to ~40% more throughput in my cases and at the same time much lower cpu consumption when broken down per transferred kilobyte (the majority of that due to saved interrupts and better cache handling). In a shared SAN others might get an additional benefit as well, because this now causes less protocol overhead. Signed-off-by: Christian Ehrhardt <ehrhardt@linux.vnet.ibm.com> Acked-by: Rik van Riel <riel@redhat.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>
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