linux-kernel - Re: [rfc 08/45] cpu alloc: x86 support

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Message-Id: <200711201459.12841.nickpiggin@yahoo.com.au>
Date:	Tue, 20 Nov 2007 14:59:12 +1100
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Christoph Lameter <clameter@....com>
Cc:	"H. Peter Anvin" <hpa@...or.com>, ak@...e.de,
	akpm@...ux-foundation.org, travis@....com,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	linux-kernel@...r.kernel.org
Subject: Re: [rfc 08/45] cpu alloc: x86 support

On Tuesday 20 November 2007 13:02, Christoph Lameter wrote:
> On Mon, 19 Nov 2007, H. Peter Anvin wrote:
> > You're making the assumption here that NUMA = large number of CPUs. This
> > assumption is flat-out wrong.
>
> Well maybe. Usually one gets to NUMA because the hardware gets too big to
> be handleed the UMA way.
>
> > On x86-64, most two-socket systems are still NUMA, and I would expect
> > that most distro kernels probably compile in NUMA.  However,
> > burning megabytes of memory on a two-socket dual-core system when we're
> > talking about tens of kilobytes used would be more than a wee bit insane.
>
> Yeah yea but the latencies are minimal making the NUMA logic too expensive
> for most loads ... If you put a NUMA kernel onto those then performance
> drops (I think someone measures 15-30%?)

Small socket count systems are going to increasingly be NUMA in future.
If CONFIG_NUMA hurts performance by that much on those systems, then the
kernel is broken IMO.
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