On 03/09/2017 08:56 AM, Mel Gorman wrote: > 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. > > Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx> Acked-by: Vlastimil Babka <vbabka@xxxxxxx> -- 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>
