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

[Joonsoo Kim]

On Mon, May 12, 2014 at 11:09:25AM +0200, Vlastimil Babka wrote:
> On 05/08/2014 07:28 AM, Joonsoo Kim wrote:
> >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?
> 
> Hm, you're right and thanks for catching that, but I think this is a
> sign of a worse and older issue than skipping a pageblock?
> When isolate_freepages_block() breaks loop due to lock contention,
> then isolate_freepages() (which called it) should also immediately
> quit its loop. Trying another pageblock in the same zone with the
> same zone->lock makes no sense here? If this is fixed, then the
> issue you're pointing out will also be fixed as next_free_pfn will
> still point to the pageblock where the break occured.
> 

Yes. It can be fixed by your approach. :)

Thanks.
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