• Mel Gorman's avatar
    mm, vmscan: only clear pgdat congested/dirty/writeback state when balanced · 631b6e08
    Mel Gorman authored
    A pgdat tracks if recent reclaim encountered too many dirty, writeback
    or congested pages.  The flags control whether kswapd writes pages back
    from reclaim context, tags pages for immediate reclaim when IO
    completes, whether processes block on wait_iff_congested and whether
    kswapd blocks when too many pages marked for immediate reclaim are
    encountered.
    
    The state is cleared in a check function with side-effects.  With the
    patch "mm, vmscan: fix zone balance check in prepare_kswapd_sleep", the
    timing of when the bits get cleared changed.  Due to the way the check
    works, it'll clear the bits if ZONE_DMA is balanced for a GFP_DMA
    allocation because it does not account for lowmem reserves properly.
    
    For the simoop workload, kswapd is not stalling when it should due to
    the premature clearing, writing pages from reclaim context like crazy
    and generally being unhelpful.
    
    This patch resets the pgdat bits related to page reclaim only when
    kswapd is going to sleep.  The comparison with simoop is then
    
                                             4.11.0-rc1            4.11.0-rc1            4.11.0-rc1
                                                vanilla           fixcheck-v2              clear-v2
    Amean    p50-Read             21670074.18 (  0.00%) 20464344.18 (  5.56%) 19786774.76 (  8.69%)
    Amean    p95-Read             25456267.64 (  0.00%) 25721423.64 ( -1.04%) 24101956.27 (  5.32%)
    Amean    p99-Read             29369064.73 (  0.00%) 30174230.76 ( -2.74%) 27691872.71 (  5.71%)
    Amean    p50-Write                1390.30 (  0.00%)     1395.28 ( -0.36%)     1011.91 ( 27.22%)
    Amean    p95-Write              412901.57 (  0.00%)    37737.74 ( 90.86%)    34874.98 ( 91.55%)
    Amean    p99-Write             6668722.09 (  0.00%)   666489.04 ( 90.01%)   575449.60 ( 91.37%)
    Amean    p50-Allocation          78714.31 (  0.00%)    86286.22 ( -9.62%)    84246.26 ( -7.03%)
    Amean    p95-Allocation         175533.51 (  0.00%)   351812.27 (-100.42%)   400058.43 (-127.91%)
    Amean    p99-Allocation         247003.02 (  0.00%)  6291171.56 (-2447.00%) 10905600.00 (-4315.17%)
    
    Read latency is improved, write latency is mostly improved but
    allocation latency is regressed.  kswapd is still reclaiming
    inefficiently, pages are being written back from writeback context and a
    host of other issues.  However, given the change, it needed to be
    spelled out why the side-effect was moved.
    
    Link: http://lkml.kernel.org/r/20170309075657.25121-3-mgorman@techsingularity.netSigned-off-by: default avatarMel Gorman <mgorman@techsingularity.net>
    Acked-by: default avatarVlastimil Babka <vbabka@suse.cz>
    Cc: Hillf Danton <hillf.zj@alibaba-inc.com>
    Cc: Johannes Weiner <hannes@cmpxchg.org>
    Cc: Shantanu Goel <sgoel01@yahoo.com>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    631b6e08
vmscan.c 111 KB