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SubjectRe: [PATCH 12/25] mm, compaction: Keep migration source private to a single compaction instance
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On 1/4/19 1:49 PM, Mel Gorman wrote:
> Due to either a fast search of the free list or a linear scan, it is
> possible for multiple compaction instances to pick the same pageblock
> for migration. This is lucky for one scanner and increased scanning for
> all the others. It also allows a race between requests on which first
> allocates the resulting free block.
>
> This patch tests and updates the pageblock skip for the migration scanner
> carefully. When isolating a block, it will check and skip if the block is
> already in use. Once the zone lock is acquired, it will be rechecked so
> that only one scanner can set the pageblock skip for exclusive use. Any
> scanner contending will continue with a linear scan. The skip bit is
> still set if no pages can be isolated in a range.

Also the skip bit will remain set even if pages *could* be isolated,
AFAICS there's no clearing after a block was finished with
nr_isolated>0. Is it intended? Note even previously it wasn't ideal,
because when pageblock was visited multiple times due to
COMPACT_CLUSTER_MAX, it would be marked with skip bit if the last visit
failed to isolate, even if the previous visits didn't.

> While this may result
> in redundant scanning, it avoids unnecessarily acquiring the zone lock
> when there are no suitable migration sources.



> 1-socket thpscale
> 4.20.0 4.20.0
> findmig-v2r15 isolmig-v2r15
> Amean fault-both-1 0.00 ( 0.00%) 0.00 * 0.00%*
> Amean fault-both-3 3505.69 ( 0.00%) 3066.68 * 12.52%*
> Amean fault-both-5 5794.13 ( 0.00%) 4298.49 * 25.81%*
> Amean fault-both-7 7663.09 ( 0.00%) 5986.99 * 21.87%*
> Amean fault-both-12 10983.36 ( 0.00%) 9324.85 ( 15.10%)
> Amean fault-both-18 13602.71 ( 0.00%) 13350.05 ( 1.86%)
> Amean fault-both-24 16145.77 ( 0.00%) 13491.77 * 16.44%*
> Amean fault-both-30 19753.82 ( 0.00%) 15630.86 * 20.87%*
> Amean fault-both-32 20616.16 ( 0.00%) 17428.50 * 15.46%*
>
> This is the first patch that shows a significant reduction in latency as
> multiple compaction scanners do not operate on the same blocks. There is
> a small increase in the success rate
>
> 4.20.0-rc6 4.20.0-rc6
> findmig-v1r4 isolmig-v1r4
> Percentage huge-3 90.58 ( 0.00%) 95.84 ( 5.81%)
> Percentage huge-5 91.34 ( 0.00%) 94.19 ( 3.12%)
> Percentage huge-7 92.21 ( 0.00%) 93.78 ( 1.71%)
> Percentage huge-12 92.48 ( 0.00%) 94.33 ( 2.00%)
> Percentage huge-18 91.65 ( 0.00%) 94.15 ( 2.72%)
> Percentage huge-24 90.23 ( 0.00%) 94.23 ( 4.43%)
> Percentage huge-30 90.17 ( 0.00%) 95.17 ( 5.54%)
> Percentage huge-32 89.72 ( 0.00%) 93.59 ( 4.32%)
>
> Compaction migrate scanned 54168306 25516488
> Compaction free scanned 800530954 87603321
>
> Migration scan rates are reduced by 52%.

Wonder how much of that is due to not clearing as pointed out above.
Also interesting how free scanned was reduced so disproportionally.

> Signed-off-by: Mel Gorman <mgorman@techsingularity.net>

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