• Mel Gorman's avatar
    mm: compaction: cache if a pageblock was scanned and no pages were isolated · bb13ffeb
    Mel Gorman authored
    When compaction was implemented it was known that scanning could
    potentially be excessive.  The ideal was that a counter be maintained for
    each pageblock but maintaining this information would incur a severe
    penalty due to a shared writable cache line.  It has reached the point
    where the scanning costs are a serious problem, particularly on
    long-lived systems where a large process starts and allocates a large
    number of THPs at the same time.
    
    Instead of using a shared counter, this patch adds another bit to the
    pageblock flags called PG_migrate_skip.  If a pageblock is scanned by
    either migrate or free scanner and 0 pages were isolated, the pageblock is
    marked to be skipped in the future.  When scanning, this bit is checked
    before any scanning takes place and the block skipped if set.
    
    The main difficulty with a patch like this is "when to ignore the cached
    information?" If it's ignored too often, the scanning rates will still be
    excessive.  If the information is too stale then allocations will fail
    that might have otherwise succeeded.  In this patch
    
    o CMA always ignores the information
    o If the migrate and free scanner meet then the cached information will
      be discarded if it's at least 5 seconds since the last time the cache
      was discarded
    o If there are a large number of allocation failures, discard the cache.
    
    The time-based heuristic is very clumsy but there are few choices for a
    better event.  Depending solely on multiple allocation failures still
    allows excessive scanning when THP allocations are failing in quick
    succession due to memory pressure.  Waiting until memory pressure is
    relieved would cause compaction to continually fail instead of using
    reclaim/compaction to try allocate the page.  The time-based mechanism is
    clumsy but a better option is not obvious.
    Signed-off-by: default avatarMel Gorman <mgorman@suse.de>
    Acked-by: default avatarRik van Riel <riel@redhat.com>
    Cc: Richard Davies <richard@arachsys.com>
    Cc: Shaohua Li <shli@kernel.org>
    Cc: Avi Kivity <avi@redhat.com>
    Acked-by: default avatarRafael Aquini <aquini@redhat.com>
    Cc: Fengguang Wu <fengguang.wu@intel.com>
    Cc: Michal Nazarewicz <mina86@mina86.com>
    Cc: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
    Cc: Kyungmin Park <kyungmin.park@samsung.com>
    Cc: Mark Brown <broonie@opensource.wolfsonmicro.com>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    bb13ffeb
internal.h 10.9 KB