Re: [PATCH 2/3] mm, vmscan: Only clear pgdat congested/dirty/writeback state when balanced

[Vlastimil Babka]

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>