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

To: Mick.Jordan@xxxxxxx, Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Subject: RE: [Xen-devel] Design question for PV superpage support
From: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>
Date: Tue, 3 Mar 2009 14:33:25 +0000 (GMT)
Cc: Dave McCracken <dcm@xxxxxxxx>, Xen Developers List <xen-devel@xxxxxxxxxxxxxxxxxxx>
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> In general, I think the guest should assume that large page 
> mappings are 
> merely an optimization that (a) might not be possible on domain start 
> due to machine memory fragmentation and (b) that this condition might 
> also occur on restore. Given these, it must always be prepared to 
> function with 4K pages, which implies that it would need to preserve 
> enough page table frame memory to be able revert from large 
> to small pages.
> Mick

Do you disagree with my assertion that use of 2MB pages is
almost always an attempt to eke out a performance improvement,
that emulating 2MB pages with fragmented 4KB pages is likely
slower than just using 4KB pages to start with, and thus
that "must always be prepared to function with 4KB pages"
should NOT occur silently (if at all)?

BTW, thinking ahead to ballooning with 2MB pages, are we prepared
to assume that a relinquished 2MB page can't be fragmented?
While this may be appealing for systems where nearly all
guests are using 2MB pages, systems where the 2MB guest is
an odd duck might suffer substantially by making that


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