• Mel Gorman's avatar
    mm: compaction: minimise the time IRQs are disabled while isolating free pages · 602605a4
    Mel Gorman authored
    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: default avatarMel Gorman <mel@csn.ul.ie>
    Acked-by: default avatarJohannes Weiner <hannes@cmpxchg.org>
    Reviewed-by: default avatarKAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujisu.com>
    Reviewed-by: default avatarMinchan Kim <minchan.kim@gmail.com>
    Acked-by: default avatarAndrea Arcangeli <aarcange@redhat.com>
    Cc: Arthur Marsh <arthur.marsh@internode.on.net>
    Cc: Clemens Ladisch <cladisch@googlemail.com>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    602605a4
compaction.c 18.6 KB