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[Xen-devel] Re: super page with live migration

To: "Zhai, Edwin" <edwin.zhai@xxxxxxxxx>
Subject: [Xen-devel] Re: super page with live migration
From: Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
Date: Fri, 26 Sep 2008 10:03:51 +0100
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On 26/9/08 08:45, "Zhai, Edwin" <edwin.zhai@xxxxxxxxx> wrote:

> As you know, we try to allocate super pages in xc_hvm_build for better
> performance in EPT case. But the same logic is missing in xc_domain_restore.
> When try to add the logic, I found it is blocked by the lazy populate
> algorithm 
> in restore -- only populating the pages received from source side rather than
> doing it at one time.
> So the result is the EPT guest has performance drop after live migration:(
> Do you have any plan to change the lazy populate algorithm? The purpose of it,
> I 
> believe, is to make restore process work without guest memory layout
> knowledge.  

Yes: If pseudo-phys page is not yet populated in target domain, AND it is
first page of a 2MB extent, AND no other pages in that extent are yet
populated, AND the next pages in the save-image stream populate that extent
in order, THEN allocate a superpage. This is made trickier by the fact that
the next 511 pages (to make the 2MB extent) might be split across a batch
boundary. So we'll have to optimistically allocate a superpage in that case,
and then shatter it if it turns out that the contiguous stream of
pseudo-phys addresses is broken in the next batch.

 -- Keir

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