On February 15, 2017 5:23 PM Mel Gorman wrote: > > From: Shantanu Goel <sgoel01@xxxxxxxxx> > > The check in prepare_kswapd_sleep needs to match the one in balance_pgdat > since the latter will return as soon as any one of the zones in the > classzone is above the watermark. This is specially important for higher > order allocations since balance_pgdat will typically reset the order to > zero relying on compaction to create the higher order pages. Without this > patch, prepare_kswapd_sleep fails to wake up kcompactd since the zone > balance check fails. > > On 4.9.7 kswapd is failing to wake up kcompactd due to a mismatch in the > zone balance check between balance_pgdat() and prepare_kswapd_sleep(). > balance_pgdat() returns as soon as a single zone satisfies the allocation > but prepare_kswapd_sleep() requires all zones to do +the same. This causes > prepare_kswapd_sleep() to never succeed except in the order == 0 case and > consequently, wakeup_kcompactd() is never called. On my machine prior to > apply this patch, the state of compaction from /proc/vmstat looked this > way after a day and a half +of uptime: > > compact_migrate_scanned 240496 > compact_free_scanned 76238632 > compact_isolated 123472 > compact_stall 1791 > compact_fail 29 > compact_success 1762 > compact_daemon_wake 0 > > After applying the patch and about 10 hours of uptime the state looks > like this: > > compact_migrate_scanned 59927299 > compact_free_scanned 2021075136 > compact_isolated 640926 > compact_stall 4 > compact_fail 2 > compact_success 2 > compact_daemon_wake 5160 > > Further notes from Mel that motivated him to pick this patch up and > resend it; > > It was observed for the simoop workload (pressures the VM similar to HADOOP) > that kswapd was failing to keep ahead of direct reclaim. The investigation > noted that there was a need to rationalise kswapd decisions to reclaim > with kswapd decisions to sleep. With this patch on a 2-socket box, there > was a 43% reduction in direct reclaim scanning. > > However, the impact otherwise is extremely negative. Kswapd reclaim > efficiency dropped from 98% to 76%. simoop has three latency-related > metrics for read, write and allocation (an anonymous mmap and fault). > > 4.10.0-rc7 4.10.0-rc7 > mmots-20170209 fixcheck-v1 > Amean p50-Read 22325202.49 ( 0.00%) 20026926.55 ( 10.29%) > Amean p95-Read 26102988.80 ( 0.00%) 27023360.00 ( -3.53%) > Amean p99-Read 30935176.53 ( 0.00%) 30994432.00 ( -0.19%) > Amean p50-Write 976.44 ( 0.00%) 1905.28 (-95.12%) > Amean p95-Write 15471.29 ( 0.00%) 36210.09 (-134.05%) > Amean p99-Write 35108.62 ( 0.00%) 479494.96 (-1265.75%) > Amean p50-Allocation 76382.61 ( 0.00%) 87603.20 (-14.69%) > Amean p95-Allocation 127777.39 ( 0.00%) 244491.38 (-91.34%) > Amean p99-Allocation 187937.39 ( 0.00%) 1745237.33 (-828.63%) > > There are also more allocation stalls. One of the largest impacts was due > to pages written back from kswapd context rising from 0 pages to 4516642 > pages during the hour the workload ran for. By and large, the patch has very > bad behaviour but easily missed as the impact on a UMA machine is negligible. > > This patch is included with the data in case a bisection leads to this area. > This patch is also a pre-requisite for the rest of the series. > > Signed-off-by: Shantanu Goel <sgoel01@xxxxxxxxx> > Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx> > --- Acked-by: Hillf Danton <hillf.zj@xxxxxxxxxxxxxxx> > mm/vmscan.c | 6 +++--- > 1 file changed, 3 insertions(+), 3 deletions(-) > > diff --git a/mm/vmscan.c b/mm/vmscan.c > index 26c3b405ef34..92fc66bd52bc 100644 > --- a/mm/vmscan.c > +++ b/mm/vmscan.c > @@ -3140,11 +3140,11 @@ static bool prepare_kswapd_sleep(pg_data_t *pgdat, int order, int classzone_idx) > if (!managed_zone(zone)) > continue; > > - if (!zone_balanced(zone, order, classzone_idx)) > - return false; > + if (zone_balanced(zone, order, classzone_idx)) > + return true; > } > > - return true; > + return false; > } > > /* > -- > 2.11.0 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
