linux-kernel - Re: [PATCH v2 2/2] mm/compaction: avoid rescanning pageblocks in isolate

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Date:	Mon, 12 May 2014 11:09:25 +0200
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Joonsoo Kim <iamjoonsoo.kim@....com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	David Rientjes <rientjes@...gle.com>,
	Hugh Dickins <hughd@...gle.com>,
	Greg Thelen <gthelen@...gle.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, Minchan Kim <minchan@...nel.org>,
	Mel Gorman <mgorman@...e.de>,
	Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
	Michal Nazarewicz <mina86@...a86.com>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Christoph Lameter <cl@...ux.com>,
	Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH v2 2/2] mm/compaction: avoid rescanning pageblocks in
 isolate_freepages

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.

> Thanks.
>

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