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

["Ian Pratt" <Ian.Pratt@xxxxxxxxxxxxx>]

> Just realize that superpage may extend the service shutdown time in
> migration process. Take a ideal convergence example for non-super page
> cases, just dozens of pages may keep dirty at last batch sent to
target
> side, and thus service shutdown phase is short. However when superpage
> is enabled, it's possible that those dozens of dirty pages are
> multiplied with a 512 factor for an extreme case where each page comes
> from different 2M super page. Then service shutdown phase can be
> longer, though not measured. Not sure how such inefficiency can be
> optimized...

Yes, ideally at the start of the migrate we'd have a new shadow control
op to turn off usage of 2MB pages in the shadow pagetable and HAP p2m
code, and to shatter any existing ones. There's no need to copy the
memory.

Ian


> 
> Thanks,
> Kevin
> 
> 
> ________________________________
> 
>       From: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx [mailto:xen-devel-
> bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of Keir Fraser
>       Sent: Saturday, September 27, 2008 6:49 PM
>       To: Ian Pratt; Zhai, Edwin
>       Cc: xen-devel@xxxxxxxxxxxxxxxxxxx
>       Subject: Re: [Xen-devel] Re: super page with live migration
> 
> 
>       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.
> 
>               Ian
> 
> 
> 
>               ----- 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.
> 
> 
> 
>               Thanks,
> 
> 
>               >  -- Keir
>               >
>               >
>               >
> 
> 
>               _______________________________________________
>               Xen-devel mailing list
>               Xen-devel@xxxxxxxxxxxxxxxxxxx
>               http://lists.xensource.com/xen-devel
> 
> 
> 
> 
> 


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