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

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Date:	Fri, 26 Jun 2015 13:22:07 +0200
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Joonsoo Kim <js1304@...il.com>
CC:	Mel Gorman <mgorman@...e.de>, 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 06/26/2015 04:14 AM, Joonsoo Kim wrote:
> 2015-06-26 3:56 GMT+09:00 Vlastimil Babka <vbabka@...e.cz>:
>>> 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?
>
> As replied to Mel's comment, as number of unmovable pageblocks, which is
> filled by movable pages due to this compaction change increases,
> possible candidate reclaimable/migratable pages from them also increase.
> So, at some time, amount of used page by free scanner and amount of
> migrated page by migration scanner would be balanced.
>
>> 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.
>
> Yes, but, next iteration will move out movable pages from that pageblock
> and freed pages will be used for further unmovable allocation.
> So, in the long term, this doesn't make much more fragmentation.

Theoretically, maybe. I guess there's not much point discussing it 
further, until there's data from experiments evaluating the long-term 
fragmentation (think of e.g. the number of mixed pageblocks you already 
checked in different experiments).

> Thanks.
>

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