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SubjectRe: [PATCH 14/25] mm, compaction: Avoid rescanning the same pageblock multiple times
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On 1/4/19 1:50 PM, Mel Gorman wrote:
> Pageblocks are marked for skip when no pages are isolated after a scan.
> However, it's possible to hit corner cases where the migration scanner
> gets stuck near the boundary between the source and target scanner. Due
> to pages being migrated in blocks of COMPACT_CLUSTER_MAX, pages that
> are migrated can be reallocated before the pageblock is complete. The
> pageblock is not necessarily skipped so it can be rescanned multiple
> times. Similarly, a pageblock with some dirty/writeback pages may fail
> to isolate and be rescanned until writeback completes which is wasteful.

^ migrate? If we failed to isolate, then it wouldn't bump nr_isolated.
Wonder if we could do better checks and not isolate pages that cannot be at the
moment migrated anyway.

>
> This patch tracks if a pageblock is being rescanned. If so, then the entire
> pageblock will be migrated as one operation. This narrows the race window
> during which pages can be reallocated during migration. Secondly, if there
> are pages that cannot be isolated then the pageblock will still be fully
> scanned and marked for skipping. On the second rescan, the pageblock skip
> is set and the migration scanner makes progress.
>
> 4.20.0 4.20.0
> finishscan-v2r15 norescan-v2r15
> Amean fault-both-3 3729.80 ( 0.00%) 2872.13 * 23.00%*
> Amean fault-both-5 5148.49 ( 0.00%) 4330.56 * 15.89%*
> Amean fault-both-7 7393.24 ( 0.00%) 6496.63 ( 12.13%)
> Amean fault-both-12 11709.32 ( 0.00%) 10280.59 ( 12.20%)
> Amean fault-both-18 16626.82 ( 0.00%) 11079.19 * 33.37%*
> Amean fault-both-24 19944.34 ( 0.00%) 17207.80 * 13.72%*
> Amean fault-both-30 23435.53 ( 0.00%) 17736.13 * 24.32%*
> Amean fault-both-32 23948.70 ( 0.00%) 18509.41 * 22.71%*
>
> 4.20.0 4.20.0
> finishscan-v2r15 norescan-v2r15
> Percentage huge-1 0.00 ( 0.00%) 0.00 ( 0.00%)
> Percentage huge-3 88.39 ( 0.00%) 96.87 ( 9.60%)
> Percentage huge-5 92.07 ( 0.00%) 94.63 ( 2.77%)
> Percentage huge-7 91.96 ( 0.00%) 93.83 ( 2.03%)
> Percentage huge-12 93.38 ( 0.00%) 92.65 ( -0.78%)
> Percentage huge-18 91.89 ( 0.00%) 93.66 ( 1.94%)
> Percentage huge-24 91.37 ( 0.00%) 93.15 ( 1.95%)
> Percentage huge-30 92.77 ( 0.00%) 93.16 ( 0.42%)
> Percentage huge-32 87.97 ( 0.00%) 92.58 ( 5.24%)
>
> The fault latency reduction is large and while the THP allocation
> success rate is only slightly higher, it's already high at this
> point of the series.
>
> Compaction migrate scanned 60718343.00 31772603.00
> Compaction free scanned 933061894.00 63267928.00

Hm I thought the order of magnitude difference between migrate and free scanned
was already gone at this point as reported in the previous 2 patches. Or is this
from different system/configuration? Anyway, encouraging result. I would expect
that after "Keep migration source private to a single compaction instance" sets
the skip bits much more early and aggressively, the rescans would not happen
anymore thanks to those, even if cached pfns were not updated.

> Migration scan rates are reduced by 48% and free scan rates are
> also reduced as the same migration source block is not being selected
> multiple times. The corner case where migration scan rates go through the
> roof due to a dirty/writeback pageblock located at the boundary of the
> migration/free scanner did not happen in this case. When it does happen,
> the scan rates multiple by factors measured in the hundreds and would be
> misleading to present.
>
> Signed-off-by: Mel Gorman <mgorman@techsingularity.net>

Acked-by: Vlastimil Babka <vbabka@suse.cz>

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