linux-kernel - Re: [RFC PATCH 00/10] redesign compaction algorithm

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Date:	Thu, 25 Jun 2015 20:56:48 +0200
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Joonsoo Kim <js1304@...il.com>, Mel Gorman <mgorman@...e.de>
CC:	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Linux Memory Management List <linux-mm@...ck.org>,
	Rik van Riel <riel@...hat.com>,
	David Rientjes <rientjes@...gle.com>,
	Minchan Kim <minchan@...nel.org>
Subject: Re: [RFC PATCH 00/10] redesign compaction algorithm

On 25.6.2015 20:14, Joonsoo Kim wrote:
>> The long-term success rate of fragmentation avoidance depends on
>> > minimsing the number of UNMOVABLE allocation requests that use a
>> > pageblock belonging to another migratetype. Once such a fallback occurs,
>> > that pageblock potentially can never be used for a THP allocation again.
>> >
>> > Lets say there is an unmovable pageblock with 500 free pages in it. If
>> > the freepage scanner uses that pageblock and allocates all 500 free
>> > pages then the next unmovable allocation request needs a new pageblock.
>> > If one is not completely free then it will fallback to using a
>> > RECLAIMABLE or MOVABLE pageblock forever contaminating it.
> Yes, I can imagine that situation. But, as I said above, we already use
> non-movable pageblock for migration scanner. While unmovable
> pageblock with 500 free pages fills, some other unmovable pageblock
> with some movable pages will be emptied. Number of freepage
> on non-movable would be maintained so fallback doesn't happen.

There's nothing that guarantees that the migration scanner will be emptying
unmovable pageblock, or am I missing something? Worse, those pageblocks would be
marked to skip by the free scanner if it isolated free pages from them, so
migration scanner would skip them.
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