linux-kernel - Re: [PATCH 0/8] Announcement: Enhanced NUMA scheduling with adaptive affinity

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Date:	Fri, 16 Nov 2012 16:59:43 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Paul Turner <pjt@...gle.com>,
	Lee Schermerhorn <Lee.Schermerhorn@...com>,
	Rik van Riel <riel@...hat.com>, Mel Gorman <mgorman@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 0/8] Announcement: Enhanced NUMA scheduling with adaptive
 affinity


* Christoph Lameter <cl@...ux.com> wrote:

> On Tue, 13 Nov 2012, Ingo Molnar wrote:
> 
> > > the pages over both nodes in use.
> >
> > I'd not go as far as to claim that to be a general rule: the 
> > correct placement depends on the system and workload 
> > specifics: how much memory is on each node, how many tasks 
> > run on each node, and whether the access patterns and 
> > working set of the tasks is symmetric amongst each other - 
> > which is not a given at all.
> >
> > Say consider a database server that executes small and large 
> > queries over a large, memory-shared database, and has worker 
> > tasks to clients, to serve each query. Depending on the 
> > nature of the queries, interleaving can easily be the wrong 
> > thing to do.
> 
> The interleaving of memory areas that have an equal amount of 
> shared accesses from multiple nodes is essential to limit the 
> traffic on the interconnect and get top performance.

That is true only if the load is symmetric.

> I guess through that in a non HPC environment where you are 
> not interested in one specific load running at top speed 
> varying contention on the interconnect and memory busses are 
> acceptable. But this means that HPC loads cannot be auto 
> tuned.

I'm not against improving these workloads (at all) - I just 
pointed out that interleaving isn't necessarily the best 
placement strategy for 'large' workloads.

Thanks,

	Ingo
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