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

To: Keir Fraser <keir.fraser@xxxxxxxxxxxxx>, Dave McCracken <dcm@xxxxxxxx>, Mick.Jordan@xxxxxxx, Xen Developers List <xen-devel@xxxxxxxxxxxxxxxxxxx>
Subject: RE: [Xen-devel] Design question for PV superpage support
From: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>
Date: Mon, 2 Mar 2009 18:48:41 +0000 (GMT)
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> From: Keir Fraser [mailto:keir.fraser@xxxxxxxxxxxxx]
> 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.

Understood.  This is a case where convenience and the primary objective
conflict.  I can't think offhand of a way to do it, but restoring
or migrating a 2MB-assumed domain into an environment where the
vast majority of 2MB pages are emulated should probably raise a
bright red flag somehow.  Or there needs to be some tool that
can at least be queried as to how many 2MB pages are being emulated.

But probably the right long-term answer is a 2MB Xen with a 2MB
Linux when applications assume/prefer 2MB pages.


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