linux-kernel - Re: [PATCH 11/11] Do not compact within a preferred zone after a compaction failure

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Date:	Wed, 24 Mar 2010 14:54:32 -0500 (CDT)
From:	Christoph Lameter <cl@...ux-foundation.org>
To:	Mel Gorman <mel@....ul.ie>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Adam Litke <agl@...ibm.com>, Avi Kivity <avi@...hat.com>,
	David Rientjes <rientjes@...gle.com>,
	Minchan Kim <minchan.kim@...il.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.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 11/11] Do not compact within a preferred zone after a
 compaction failure

On Wed, 24 Mar 2010, Mel Gorman wrote:

> > > What I was thinking at the time was that compact_resume was stored in struct
> > > zone - i.e. that is where it is recorded.
> >
> > Ok adding a dozen or more words here may be useful.
> >
>
> In the event of compaction followed by an allocation failure, this patch
> defers further compaction in the zone for a period of time. The zone that
> is deferred is the first zone in the zonelist - i.e. the preferred zone.
> To defer compaction in the other zones, the information would need to
> be stored in the zonelist or implemented similar to the zonelist_cache.
> This would impact the fast-paths and is not justified at this time.
>
> ?

Ok.

> > There are frequent uses of HZ/10 as well especially in vmscna.c. A longer
> > time may be better? HZ/50 looks like an interval for writeout. But this
> > is related to reclaim?
> >
>
> HZ/10 is somewhat of an arbitrary choice as well and there isn't data on
> which is better and which is worse. If the zone is full of dirty data, then
> HZ/10 makes sense for IO. If it happened to be mainly clean cache but under
> heavy memory pressure, then reclaim would be a relatively fast event and a
> shorter wait makes sense of HZ/50.
>
> Thing is, if we start with a short timer and it's too short, COMPACTFAIL
> will be growing steadily. If we choose a long time and it's too long, there
> is no counter to indicate it was a bad choice. Hence, I'd prefer the short
> timer to start with and ideally resume compaction after some event in the
> future rather than depending on time.
>
> Does that make sense?

Yes.

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