Re: [PATCH v3 7/7] mm/compaction: replace compaction deferring with compaction limit

[Vlastimil Babka]

On 12/03/2015 08:11 AM, Joonsoo Kim wrote:
> Compaction deferring effectively reduces compaction overhead if
> compaction success isn't expected. But, it is implemented that
> skipping a number of compaction requests until compaction is re-enabled.
> Due to this implementation, unfortunate compaction requestor will get
> whole compaction overhead unlike others have zero overhead. And, after
> deferring start to work, even if compaction success possibility is
> restored, we should skip to compaction in some number of times.
>
> This patch try to solve above problem by using compaction limit.
> Instead of imposing compaction overhead to one unfortunate requestor,
> compaction limit distributes overhead to all compaction requestors.
> All requestors have a chance to migrate some amount of pages and
> after limit is exhausted compaction will be stopped. This will fairly
> distributes overhead to all compaction requestors. And, because we don't
> defer compaction request, someone will succeed to compact as soon as
> possible if compaction success possiblility is restored.
>
> Following is whole workflow enabled by this change.
>
> - if sync compaction fails, compact_order_failed is set to current order
> - if it fails again, compact_defer_shift is adjusted
> - with positive compact_defer_shift, migration_scan_limit is assigned and
> compaction limit is activated
> - if compaction limit is activated, compaction would be stopped when
> migration_scan_limit is exhausted
> - when success, compact_defer_shift and compact_order_failed is reset and
> compaction limit is deactivated
> - compact_defer_shift can be grown up to COMPACT_MAX_DEFER_SHIFT
>
> Most of changes are mechanical ones to remove compact_considered which
> is not needed now. Note that, after restart, compact_defer_shift is
> subtracted by 1 to avoid invoking __reset_isolation_suitable()
> repeatedly.
>
> I tested this patch on my compaction benchmark and found that high-order
> allocation latency is evenly distributed and there is no latency spike
> in the situation where compaction success isn't possible.
>
> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>

Looks fine overal, looking forward to next version :) (due to changes 
expected in preceding patches, I didn't review the code fully now).

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