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

To: Keir Fraser <keir.fraser@xxxxxxxxxxxxx>, 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 09:52:04 +0100
Cc: xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxx>, Bill Burns <bburns@xxxxxxxxxx>
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On 13/8/08 09:47, "Keir Fraser" <keir.fraser@xxxxxxxxxxxxx> wrote:

> 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.

Put another way, at 512 CPUs the cpumasks would incur an overhead of <2% of
total memory. It's only really beyond that threshold that I'd be concerned.
The fact is it'll be a good while before 512 CPUs is concerning us, and
we'll have plenty of other scalability concerns, no doubt, by that point.

 -- Keir

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