Hi, On Wed, Feb 15, 2017 at 09:22:45AM +0000, 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> Hmm, I don't understand why we should bind wakeup_kcompactd to kswapd's short sleep point where every eligible zones are balanced. What's the correlation between them? Can't we wake up kcompactd once we found a zone has enough free pages above high watermark like this? diff --git a/mm/vmscan.c b/mm/vmscan.c index 26c3b405ef34..f4f0ad0e9ede 100644 --- a/mm/vmscan.c +++ b/mm/vmscan.c @@ -3346,13 +3346,6 @@ static void kswapd_try_to_sleep(pg_data_t *pgdat, int alloc_order, int reclaim_o * that pages and compaction may succeed so reset the cache. */ reset_isolation_suitable(pgdat); - - /* - * We have freed the memory, now we should compact it to make - * allocation of the requested order possible. - */ - wakeup_kcompactd(pgdat, alloc_order, classzone_idx); - remaining = schedule_timeout(HZ/10); /* @@ -3451,6 +3444,14 @@ static int kswapd(void *p) bool ret; kswapd_try_sleep: + /* + * We have freed the memory, now we should compact it to make + * allocation of the requested order possible. + */ + if (alloc_order > 0 && zone_balanced(zone, reclaim_order, + classzone_idx)) + wakeup_kcompactd(pgdat, alloc_order, classzone_idx); + kswapd_try_to_sleep(pgdat, alloc_order, reclaim_order, classzone_idx); -- 2.7.4 -- 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>
