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Re: [Xen-devel] Design question for PV superpage support

To: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>, Dave McCracken <dcm@xxxxxxxx>, "Mick.Jordan@xxxxxxx" <Mick.Jordan@xxxxxxx>, Xen Developers List <xen-devel@xxxxxxxxxxxxxxxxxxx>
Subject: Re: [Xen-devel] Design question for PV superpage support
From: Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
Date: Mon, 02 Mar 2009 18:30:01 +0000
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On 02/03/2009 18:03, "Dan Magenheimer" <dan.magenheimer@xxxxxxxxxx> wrote:

>> From: Keir Fraser [mailto:keir.fraser@xxxxxxxxxxxxx]
>> It'd be nice to fall back to the case of not being able to
>> guarantee all 2MB
>> extents are aligned and contiguous. So for example being able
>> to migrate to
>> or restore on a system that currently doesn't have enough
>> contiguous memory.
> Well, yes and no.  I believe the ONLY reason to use 2MB
> pages is to achieve a significant performance advantage.
> And I suspect emulating 2MB "virtual pages" on 4KB physical
> pages will perform at least slightly worse than just
> 4KB-on-4KB, true?

If you make this constraint then you risk creating domains that you cannot
always conveniently restore. Obviously you would allocate 2MB extents
wherever possible, since that is the whole point of this drawn out exercise.

 -- Keir

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