linux-kernel - Re: [PATCH 10/11] Direct compact when a high-order allocation fails

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Date:	Fri, 19 Mar 2010 10:10:16 +0000
From:	Mel Gorman <mel@....ul.ie>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Adam Litke <agl@...ibm.com>, Avi Kivity <avi@...hat.com>,
	David Rientjes <rientjes@...gle.com>,
	Rik van Riel <riel@...hat.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [PATCH 10/11] Direct compact when a high-order allocation fails

On Fri, Mar 19, 2010 at 03:31:27PM +0900, KOSAKI Motohiro wrote:
> > Viewpoint 1. Unnecessary IO
> > 
> > isolate_pages() for lumpy reclaim frequently grab very young page. it is often
> > still dirty. then, pageout() is called much.
> > 
> > Unfortunately, page size grained io is _very_ inefficient. it can makes lots disk
> > seek and kill disk io bandwidth.
> > 
> > 
> > Viewpoint 2. Unevictable pages 
> > 
> > isolate_pages() for lumpy reclaim can pick up unevictable page. it is obviously
> > undroppable. so if the zone have plenty mlocked pages (it is not rare case on
> > server use case), lumpy reclaim can become very useless.
> > 
> > 
> > Viewpoint 3. GFP_ATOMIC allocation failure
> > 
> > Obviously lumpy reclaim can't help GFP_ATOMIC issue.
> > 
> > 
> > Viewpoint 4. reclaim latency
> > 
> > reclaim latency directly affect page allocation latency. so if lumpy reclaim with
> > much pageout io is slow (often it is), it affect page allocation latency and can
> > reduce end user experience.
> 
> Viewpoint 5. end user surprising
> 
> lumpy reclaim can makes swap-out even though the system have lots free
> memory. end users very surprised it and they can think it is bug.
> 
> Also, this swap activity easyly confuse that an administrator decide when
> install more memory into the system.
> 

Compaction in this case is a lot less surprising. If there is enough free
memory, compaction will trigger automatically without any reclaim.

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab
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