linux-kernel - Re: [RFC PATCH 2/3] mm: more aggressive page stealing for UNMOVABLE allocations

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Date:	Tue, 9 Dec 2014 09:47:18 +0000
From:	Mel Gorman <mgorman@...e.de>
To:	Minchan Kim <minchan@...nel.org>
Cc:	Vlastimil Babka <vbabka@...e.cz>, linux-mm@...ck.org,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	linux-kernel@...r.kernel.org, Rik van Riel <riel@...hat.com>,
	David Rientjes <rientjes@...gle.com>
Subject: Re: [RFC PATCH 2/3] mm: more aggressive page stealing for UNMOVABLE
 allocations

On Tue, Dec 09, 2014 at 12:09:40PM +0900, Minchan Kim wrote:
> On Thu, Dec 04, 2014 at 06:12:57PM +0100, Vlastimil Babka wrote:
> > When allocation falls back to stealing free pages of another migratetype,
> > it can decide to steal extra pages, or even the whole pageblock in order to
> > reduce fragmentation, which could happen if further allocation fallbacks
> > pick a different pageblock. In try_to_steal_freepages(), one of the situations
> > where extra pages are stolen happens when we are trying to allocate a
> > MIGRATE_RECLAIMABLE page.
> > 
> > However, MIGRATE_UNMOVABLE allocations are not treated the same way, although
> > spreading such allocation over multiple fallback pageblocks is arguably even
> > worse than it is for RECLAIMABLE allocations. To minimize fragmentation, we
> > should minimize the number of such fallbacks, and thus steal as much as is
> > possible from each fallback pageblock.
> 
> Fair enough.
> 

Just to be absolutly sure, check that data and see what the number of
MIGRATE_UNMOVABLE blocks looks like over time. Make sure it's not just
continually growing. MIGRATE_RECLAIMABLE and MIGRATE_MOVABLE blocks were
expected to be freed if the system was aggressively reclaimed but the same
is not be true of MIGRATE_UNMOVABLE. Even if all processes are
aggressively reclaimed for example, the page tables are still there.

-- 
Mel Gorman
SUSE Labs
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