linux-kernel - Re: [PATCH 1/2] mm: page allocator: Initialise ZLC for first zone eligible for zone

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Message-ID: <20110718160552.GB5349@suse.de>
Date:	Mon, 18 Jul 2011 17:05:52 +0100
From:	Mel Gorman <mgorman@...e.de>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Minchan Kim <minchan.kim@...il.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] mm: page allocator: Initialise ZLC for first zone
 eligible for zone_reclaim

On Mon, Jul 18, 2011 at 09:56:31AM -0500, Christoph Lameter wrote:
> On Fri, 15 Jul 2011, Mel Gorman wrote:
> 
> > Currently the zonelist cache is setup only after the first zone has
> > been considered and zone_reclaim() has been called. The objective was
> > to avoid a costly setup but zone_reclaim is itself quite expensive. If
> > it is failing regularly such as the first eligible zone having mostly
> > mapped pages, the cost in scanning and allocation stalls is far higher
> > than the ZLC initialisation step.
> 
> Would it not be easier to set zlc_active and allowednodes based on the
> zone having an active ZLC at the start of get_pages()?
> 

What do you mean by a zones active ZLC? zonelists are on a per-node,
not a per-zone basis (see node_zonelist) so a zone doesn't have an
active ZLC as such. If the zlc_active is set at the beginning of
get_page_from_freelist(), it implies that we are calling zlc_setup()
even when the watermarks are met which is unnecessary.

> Buffered_rmqueue is handling the situation of a zone with an ZLC in a
> weird way right now since it ignores the (potentially existing) ZLC
> for the first pass.

Where does buffered_rmqueue() refer to a zonelist_cache?

> zlc_setup() does a lot of things. So that is because
> there is a performance benefit?
> 

I do not understand this question. Are you asking if zonelist_cache
has a performance benefit? The answer is "yes" because you can see
how the performance when zone_reclaim degrades when it is not used
for the first zone.

-- 
Mel Gorman
SUSE Labs
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