Re: [PATCH -mm v2] mm: have order > 0 compaction start off whereit left

[Kamezawa Hiroyuki]

(2012/06/29 2:55), Rik van Riel wrote:
> Order > 0 compaction stops when enough free pages of the correct
> page order have been coalesced. When doing subsequent higher order
> allocations, it is possible for compaction to be invoked many times.
>
> However, the compaction code always starts out looking for things to
> compact at the start of the zone, and for free pages to compact things
> to at the end of the zone.
>
> This can cause quadratic behaviour, with isolate_freepages starting
> at the end of the zone each time, even though previous invocations
> of the compaction code already filled up all free memory on that end
> of the zone.
>
> This can cause isolate_freepages to take enormous amounts of CPU
> with certain workloads on larger memory systems.
>
> The obvious solution is to have isolate_freepages remember where
> it left off last time, and continue at that point the next time
> it gets invoked for an order > 0 compaction. This could cause
> compaction to fail if cc->free_pfn and cc->migrate_pfn are close
> together initially, in that case we restart from the end of the
> zone and try once more.
>
> Forced full (order == -1) compactions are left alone.
>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Mel Gorman <mel@xxxxxxxxx>
> Reported-by: Jim Schutt <jaschut@xxxxxxxxxx>
> Signed-off-by: Rik van Riel <riel@xxxxxxxxxx>

Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>


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