compaction_alloc() isolates free pages to be used as migration targets.
While its scanning, IRQs are disabled on the mistaken assumption the
scanning should be short. Analysis showed that IRQs were in fact being
disabled for substantial time. A simple test was run using large
anonymous mappings with transparent hugepage support enabled to trigger
frequent compactions. A monitor sampled what the worst IRQ-off latencies
were and a post-processing tool found the following;
Total sampled time IRQs off (not real total time): 22355
Event compaction_alloc..compaction_alloc 8409 us count 1
Event compaction_alloc..compaction_alloc 7341 us count 1
Event compaction_alloc..compaction_alloc 2463 us count 1
Event compaction_alloc..compaction_alloc 2054 us count 1
Event shrink_inactive_list..shrink_zone 1864 us count 1
Event shrink_inactive_list..shrink_zone 88 us count 1
Event save_args..call_softirq 36 us count 1
Event save_args..call_softirq 35 us count 2
Event __make_request..__blk_run_queue 24 us count 1
Event __alloc_pages_nodemask..__alloc_pages_nodemask 6 us count 1
i.e. compaction is disabled IRQs for a prolonged period of time - 8ms in
one instance. The full report generated by the tool can be found at
http://www.csn.ul.ie/~mel/postings/minfree-20110225/irqsoff-vanilla-micro.report
This patch reduces the time IRQs are disabled by simply disabling IRQs at
the last possible minute. An updated IRQs-off summary report then looks
like;
Total sampled time IRQs off (not real total time): 5493
Event shrink_inactive_list..shrink_zone 1596 us count 1
Event shrink_inactive_list..shrink_zone 1530 us count 1
Event shrink_inactive_list..shrink_zone 956 us count 1
Event shrink_inactive_list..shrink_zone 541 us count 1
Event shrink_inactive_list..shrink_zone 531 us count 1
Event split_huge_page..add_to_swap 232 us count 1
Event save_args..call_softirq 36 us count 1
Event save_args..call_softirq 35 us count 2
Event __wake_up..__wake_up 1 us count 1
A full report is again available at
http://www.csn.ul.ie/~mel/postings/minfree-20110225/irqsoff-minimiseirq-free-v1r4-micro.report
As should be obvious, IRQ disabled latencies due to compaction are
almost elimimnated for this particular test.
[aarcange@redhat.com: Fix initialisation of isolated]
Signed-off-by: Mel Gorman <mel@csn.ul.ie>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujisu.com>
Reviewed-by: Minchan Kim <minchan.kim@gmail.com>
Acked-by: Andrea Arcangeli <aarcange@redhat.com>
Cc: Arthur Marsh <arthur.marsh@internode.on.net>
Cc: Clemens Ladisch <cladisch@googlemail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
602605a42e