linux-kernel - Re: excessive CPU utilization by isolate

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Message-ID: <4FEBAB92.4090206@kernel.org>
Date:	Thu, 28 Jun 2012 09:55:46 +0900
From:	Minchan Kim <minchan@...nel.org>
To:	Rik van Riel <riel@...hat.com>
CC:	Jim Schutt <jaschut@...dia.gov>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mel@....ul.ie>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	"ceph-devel@...r.kernel.org" <ceph-devel@...r.kernel.org>
Subject: Re: excessive CPU utilization by isolate_freepages?

On 06/28/2012 09:28 AM, Rik van Riel wrote:

> On 06/27/2012 07:59 PM, Minchan Kim wrote:
> 
>> I doubt compaction try to migrate continuously although we have no
>> free memory.
>> Could you apply this patch and retest?
>>
>> https://lkml.org/lkml/2012/6/21/30
> 
> Another possibility is that compaction is succeeding every time,
> but since we always start scanning all the way at the beginning
> and end of each zone, we waste a lot of CPU time rescanning the
> same pages (that we just filled up with moved pages) to see if
> any are free.


It does make sense.

> 
> In short, due to the way compaction behaves right now,
> compaction + isolate_freepages are essentially quadratic.
> 
> What we need to do is remember where we left off after a
> successful compaction, so we can continue the search there
> at the next invocation.
> 


Good idea.
It could enhance parallel compaction, too.
Of course, if we can't meet the goal, we need loop around from start/end of zone.

-- 
Kind regards,
Minchan Kim
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