linux-kernel - Re: [RFC PATCH 00/20] Cleanup and optimise the page allocator

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Date:	Tue, 24 Feb 2009 14:32:18 +0000
From:	Mel Gorman <mel@....ul.ie>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Linux Memory Management List <linux-mm@...ck.org>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Rik van Riel <riel@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Nick Piggin <npiggin@...e.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Lin Ming <ming.m.lin@...el.com>,
	Zhang Yanmin <yanmin_zhang@...ux.intel.com>
Subject: Re: [RFC PATCH 00/20] Cleanup and optimise the page allocator

On Mon, Feb 23, 2009 at 06:49:48PM +0100, Andi Kleen wrote:
> > hmm, it would be ideal but I haven't looked too closely at how it could
> > be implemented. I thought first you could just associate a zonelist with
> 
> Yes like that. This was actually discussed during the initial cpuset
> implementation. I thought back then it would be better to do it
> elsewhere, but changed my mind later when I saw the impact on the
> fast path.
> 

Back then there would have been other anomolies as well such as
MPOL_BIND using zones in the wrong order. Zeroing would still have
dominated the cost of the allocation and slab would hide other details.
Hindsight is 20/20 and all that.

Right now, I don't think cpusets are a dominant factor for most setups but
I'm open to being convinced otherwise. For now, I'm happy if it's just shoved
a bit more to the side in the non-cpuset case. Like the CPU cache hot/cold
path, it might be best to leave it for a second or third pass and tackle
the low-lying fruit for the first pass.

> > the cpuset but you'd need one for each node allowed by the cpuset so it
> > could get quite large. Then again, it might be worthwhile if cpusets
> 
> Yes you would need one per node, but that's not a big problem because
> systems with lots of nodes are also expected to have lots of memory.
> Most systems have a very small number of nodes.
> 

That's a fair point on the memory consumption. There might be issues
with the cache consumption but if the cpuset is being heavily used for an
allocation-intensive workload then it probably will not be noticeable.

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