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

To: Ian Pratt <Ian.Pratt@xxxxxxxxxxxxx>, <edwin.zhai@xxxxxxxxx>
Subject: Re: [Xen-devel] Re: super page with live migration
From: Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
Date: Sat, 27 Sep 2008 11:49:10 +0100
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Yes, quite apart from anything else, you’re just punting the problem of detecting superpages (or candidates for superpages) to the sender. I’m pretty confident it won’t be slow and, for a live migration, the superpage logic in the restore logic is only really going to get kicked in the first batch (we could even add that as an extra condition) so won’t extend ‘dead time’ at all.

 -- Keir

On 27/9/08 11:06, "Ian Pratt" <Ian.Pratt@xxxxxxxxxxxxx> wrote:

I don't think the proposed logic is particularly tricky, and it won't be slow - the initial test of looking for the first page in a 2.MB extent acts as a good filter.

It will work better than mmarking super pages on the sender.


----- Original Message -----
From: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx <xen-devel-bounces@xxxxxxxxxxxxxxxxxxx>
To: Keir Fraser
Cc: xen-devel@xxxxxxxxxxxxxxxxxxx <xen-devel@xxxxxxxxxxxxxxxxxxx>
Sent: Sat Sep 27 10:19:15 2008
Subject: [Xen-devel] Re: super page with live migration

Keir Fraser wrote:
> 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.

It's really tricky logic, and may make the migration process longer:(

Maybe the flag the start-of-a-superpage as Tim said can make it easier.


>  -- Keir

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