Date: Fri, 29 Jun 2012 11:02:58 +0100
From: Mel Gorman <mel@csn.ul.ie>
To: Rik van Riel <riel@redhat.com>
Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>, jaschut@sandia.gov,
        minchan@kernel.org, kamezawa.hiroyu@jp.fujitsu.com
Subject: Re: [PATCH -mm v2] mm: have order > 0 compaction start off where it
 left
Message-ID: <20120629100258.GA13141@csn.ul.ie>
References: <20120628135520.0c48b066@annuminas.surriel.com>
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On Thu, Jun 28, 2012 at 01:55:20PM -0400, Rik van Riel wrote:
> Order > 0 compaction stops when enough free pages of the correct
> page order have been coalesced. When doing subsequent higher order
> allocations, it is possible for compaction to be invoked many times.
> 
> However, the compaction code always starts out looking for things to
> compact at the start of the zone, and for free pages to compact things
> to at the end of the zone.
> 
> This can cause quadratic behaviour, with isolate_freepages starting
> at the end of the zone each time, even though previous invocations
> of the compaction code already filled up all free memory on that end
> of the zone.
> 
> This can cause isolate_freepages to take enormous amounts of CPU
> with certain workloads on larger memory systems.
> 
> The obvious solution is to have isolate_freepages remember where
> it left off last time, and continue at that point the next time
> it gets invoked for an order > 0 compaction. This could cause
> compaction to fail if cc->free_pfn and cc->migrate_pfn are close
> together initially, in that case we restart from the end of the
> zone and try once more.
> 
> Forced full (order == -1) compactions are left alone.
> 
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Mel Gorman <mel@csn.ul.ie>
> Reported-by: Jim Schutt <jaschut@sandia.gov>
> Signed-off-by: Rik van Riel <riel@redhat.com>
> ---
> v2: implement Mel's suggestions, handling wrap-around etc
> 

I have not tested it myself but it looks correct! Thanks very much.

Acked-by: Mel Gorman <mel@csn.ul.ie>

-- 
Mel Gorman
SUSE Labs
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