Re: [PATCH v2 2/2] mm/compaction: avoid rescanning pageblocks in isolate_freepages

[Joonsoo Kim]

On Wed, May 07, 2014 at 02:09:10PM +0200, Vlastimil Babka wrote:
> The compaction free scanner in isolate_freepages() currently remembers PFN of
> the highest pageblock where it successfully isolates, to be used as the
> starting pageblock for the next invocation. The rationale behind this is that
> page migration might return free pages to the allocator when migration fails
> and we don't want to skip them if the compaction continues.
> 
> Since migration now returns free pages back to compaction code where they can
> be reused, this is no longer a concern. This patch changes isolate_freepages()
> so that the PFN for restarting is updated with each pageblock where isolation
> is attempted. Using stress-highalloc from mmtests, this resulted in 10%
> reduction of the pages scanned by the free scanner.

Hello,

Although this patch could reduce page scanned, it is possible to skip
scanning fresh pageblock. If there is zone lock contention and we are on
asyn compaction, we stop scanning this pageblock immediately. And
then, we will continue to scan next pageblock. With this patch,
next_free_pfn is updated in this case, so we never come back again to this
pageblock. Possibly this makes compaction success rate low, doesn't
it?

Thanks.

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