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Re: [Xen-devel] RFC: large system support - 128 CPUs

To: Jan Beulich <jbeulich@xxxxxxxxxx>, Tim Deegan <Tim.Deegan@xxxxxxxxxx>
Subject: Re: [Xen-devel] RFC: large system support - 128 CPUs
From: Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
Date: Wed, 13 Aug 2008 09:47:52 +0100
Cc: xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxx>, Bill Burns <bburns@xxxxxxxxxx>
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On 13/8/08 09:45, "Jan Beulich" <jbeulich@xxxxxxxxxx> wrote:

> But the cpumask-in-page_info is a scalability concern, too - systems with
> many CPUs will tend to have a lot of memory, and the growing overhead
> of the page_info array may become an issue then, too. Page clustering
> may be an option to reduce/eliminate the growth, though I didn't spend
> much thought on this or possible alternatives.

An extra 8 bytes per page per 64 CPUs is hardly a concern I think. We're
talking an overhead of 32 bytes per megabyte per CPU. The concern over
growing page_info array with growing memory is fallacious -- the overhead is
a constant fraction of total memory, if #CPUs is held constant.

 -- Keir

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