• Mike Galbraith's avatar
    sched: Strengthen buddies and mitigate buddy induced latencies · f685ceac
    Mike Galbraith authored
    This patch restores the effectiveness of LAST_BUDDY in preventing
    pgsql+oltp from collapsing due to wakeup preemption. It also
    switches LAST_BUDDY to exclusively do what it does best, namely
    mitigate the effects of aggressive wakeup preemption, which
    improves vmark throughput markedly, and restores mysql+oltp
    scalability.
    
    Since buddies are about scalability, enable them beginning at the
    point where we begin expanding sched_latency, namely
    sched_nr_latency. Previously, buddies were cleared aggressively,
    which seriously reduced their effectiveness. Not clearing
    aggressively however, produces a small drop in mysql+oltp
    throughput immediately after peak, indicating that LAST_BUDDY is
    actually doing some harm. This is right at the point where X on the
    desktop in competition with another load wants low latency service.
    Ergo, do not enable until we need to scale.
    
    To mitigate latency induced by buddies, or by a task just missing
    wakeup preemption, check latency at tick time.
    
    Last hunk prevents buddies from stymieing BALANCE_NEWIDLE via
    CACHE_HOT_BUDDY.
    
    Supporting performance tests:
    
     tip   = v2.6.32-rc5-1497-ga525b32
     tipx  = NO_GENTLE_FAIR_SLEEPERS NEXT_BUDDY granularity knobs = 31 knobs + 31 buddies
     tip+x = NO_GENTLE_FAIR_SLEEPERS granularity knobs = 31 knobs
    
    (Three run averages except where noted.)
    
     vmark:
     ------
     tip           108466 messages per second
     tip+          125307 messages per second
     tip+x         125335 messages per second
     tipx          117781 messages per second
     2.6.31.3      122729 messages per second
    
     mysql+oltp:
     -----------
     clients          1        2        4        8       16       32       64        128    256
     ..........................................................................................
     tip        9949.89 18690.20 34801.24 34460.04 32682.88 30765.97 28305.27 25059.64 19548.08
     tip+      10013.90 18526.84 34900.38 34420.14 33069.83 32083.40 30578.30 28010.71 25605.47
     tipx       9698.71 18002.70 34477.56 33420.01 32634.30 31657.27 29932.67 26827.52 21487.18
     2.6.31.3   8243.11 18784.20 34404.83 33148.38 31900.32 31161.90 29663.81 25995.94 18058.86
    
     pgsql+oltp:
     -----------
     clients          1        2        4        8       16       32       64      128      256
     ..........................................................................................
     tip       13686.37 26609.25 51934.28 51347.81 49479.51 45312.65 36691.91 26851.57 24145.35
     tip+ (1x) 13907.85 27135.87 52951.98 52514.04 51742.52 50705.43 49947.97 48374.19 46227.94
     tip+x     13906.78 27065.81 52951.19 52542.59 52176.11 51815.94 50838.90 49439.46 46891.00
     tipx      13742.46 26769.81 52351.99 51891.73 51320.79 50938.98 50248.65 48908.70 46553.84
     2.6.31.3  13815.35 26906.46 52683.34 52061.31 51937.10 51376.80 50474.28 49394.47 47003.25
    Signed-off-by: default avatarMike Galbraith <efault@gmx.de>
    Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
    LKML-Reference: <new-submission>
    Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
    f685ceac
sched_fair.c 48 KB