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

To: "Tim Deegan" <Tim.Deegan@xxxxxxxxxx>, "Keir Fraser" <keir.fraser@xxxxxxxxxxxxx>
Subject: Re: [Xen-devel] RFC: large system support - 128 CPUs
From: "Jan Beulich" <jbeulich@xxxxxxxxxx>
Date: Wed, 13 Aug 2008 09:45:13 +0100
Cc: xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxx>, Bill Burns <bburns@xxxxxxxxxx>
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>>> Keir Fraser <keir.fraser@xxxxxxxxxxxxx> 13.08.08 10:26 >>>
>On 13/8/08 09:22, "Tim Deegan" <Tim.Deegan@xxxxxxxxxx> wrote:
>> At 09:21 +0100 on 13 Aug (1218619274), Jan Beulich wrote:
>>> Both seem to be hacks to get to 128 CPUs, without consideration of how
>>> to go beyond that
>> I think the shadow_page_info one is a general fix for my implicit
>> assumption that sizeof(cpumask_t) == sizeof (long).
>Do some fields after the cpumask need to line up in both structures? Placing
>a dummy cpumask in the shadow_page structure might make most sense.
>For the other one I'll have to think a bit. The need for GDT entries per CPU
>currently obviously means scaling much past a few hundred CPUs is going to
>be difficult.

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.


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