• Mel Gorman's avatar
    mm: compaction: clear PG_migrate_skip based on compaction and reclaim activity · 62997027
    Mel Gorman authored
    Compaction caches if a pageblock was scanned and no pages were isolated so
    that the pageblocks can be skipped in the future to reduce scanning.  This
    information is not cleared by the page allocator based on activity due to
    the impact it would have to the page allocator fast paths.  Hence there is
    a requirement that something clear the cache or pageblocks will be skipped
    forever.  Currently the cache is cleared if there were a number of recent
    allocation failures and it has not been cleared within the last 5 seconds.
    Time-based decisions like this are terrible as they have no relationship
    to VM activity and is basically a big hammer.
    
    Unfortunately, accurate heuristics would add cost to some hot paths so
    this patch implements a rough heuristic.  There are two cases where the
    cache is cleared.
    
    1. If a !kswapd process completes a compaction cycle (migrate and free
       scanner meet), the zone is marked compact_blockskip_flush. When kswapd
       goes to sleep, it will clear the cache. This is expected to be the
       common case where the cache is cleared. It does not really matter if
       kswapd happens to be asleep or going to sleep when the flag is set as
       it will be woken on the next allocation request.
    
    2. If there have been multiple failures recently and compaction just
       finished being deferred then a process will clear the cache and start a
       full scan.  This situation happens if there are multiple high-order
       allocation requests under heavy memory pressure.
    
    The clearing of the PG_migrate_skip bits and other scans is inherently
    racy but the race is harmless.  For allocations that can fail such as THP,
    they will simply fail.  For requests that cannot fail, they will retry the
    allocation.  Tests indicated that scanning rates were roughly similar to
    when the time-based heuristic was used and the allocation success rates
    were similar.
    Signed-off-by: default avatarMel Gorman <mgorman@suse.de>
    Cc: Rik van Riel <riel@redhat.com>
    Cc: Richard Davies <richard@arachsys.com>
    Cc: Shaohua Li <shli@kernel.org>
    Cc: Avi Kivity <avi@redhat.com>
    Cc: Rafael Aquini <aquini@redhat.com>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    62997027
vmscan.c 100 KB