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

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Message-ID: <20100325101653.GN2024@csn.ul.ie>
Date:	Thu, 25 Mar 2010 10:16:54 +0000
From:	Mel Gorman <mel@....ul.ie>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...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>,
	Minchan Kim <minchan.kim@...il.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.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 Thu, Mar 25, 2010 at 06:50:21PM +0900, KAMEZAWA Hiroyuki wrote:
> On Thu, 25 Mar 2010 09:48:26 +0000
> Mel Gorman <mel@....ul.ie> wrote:
> 
> > > In that case, compact_finished() can't
> > > find there is a free chunk and do more work.  How about using a function like
> > > 	 free_pcppages_bulk(zone, pcp->batch, pcp);
> > > to bypass pcp list and freeing pages at once ?
> > > 
> > 
> > I think you mean to drain the PCP lists while compaction is happening
> > but is it justified? It's potentially a lot of IPI calls just to check
> > if compaction can finish a little earlier. If the pages on the PCP lists
> > are making that much of a difference to high-order page availability, it
> > implies that the zone is pretty full and it's likely that compaction was
> > avoided and we direct reclaimed.
> > 
> Ah, sorry for my short word again. I mean draining "local" pcp list because
> a thread which run direct-compaction freed pages. IPI is not necessary and
> overkill.
> 

Ah, I see now. There are two places that pages get freed.  release_freepages()
at the end of compaction when it's too late for compact_finished() to be
helped and within migration itself. Migration frees with either
free_page() or more commonly put_page() with put_page() being the most
frequently used. As free_page() is called on failure to migrate (rare),
there is little help in changing it and I'd rather not modify how
put_page() works.

I could add a variant of drain_local_pages() that drains just the local PCP of
a given zone before compact_finished() is called. The cost would be a doubling
of the number of times zone->lock is taken to do the drain. Is it
justified? It seems overkill to me to take the zone->lock just in case
compaction can finish a little earlier. It feels like it would be adding
a guaranteed cost for a potential saving.

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