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

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Date:	Mon, 19 Nov 2007 18:18:42 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Christoph Lameter <clameter@....com>
CC:	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

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%?)
> 

How do you handle this memory, in the first place?  Do you allocate the 
whole 2 MB for the particular CPU, or do you reclaim the upper part of 
the large page?  (I haven't dug far enough into the source to tell.)

	-hpa
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