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

To: Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
Subject: Re: [Xen-devel] Design question for PV superpage support
From: Mick Jordan <Mick.Jordan@xxxxxxx>
Date: Mon, 02 Mar 2009 10:46:03 -0800
Cc: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>, Dave McCracken <dcm@xxxxxxxx>, Xen Developers List <xen-devel@xxxxxxxxxxxxxxxxxxx>
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On 03/02/09 10:30, Keir Fraser wrote:
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.
Indeed, performance is the issue, less TLB misses. I'm happy to use 2MB pages when I can and fall back on 4K when I can't. I just want Xen not to fall over and save/restore to work.


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