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Re: [Xen-devel] [PATCH 2 of 2 RFC] xl: allow for moving the domain's memory when changing vcpu affinity

On Fri, 2012-07-06 at 14:30 +0100, George Dunlap wrote:
> On 06/07/12 14:25, Ian Campbell wrote:
> > On Fri, 2012-07-06 at 13:53 +0100, George Dunlap wrote:
> >> On 06/07/12 10:54, Dario Faggioli wrote:
> >>> By introducing a new '-M' option to the `xl vcpu-pin' command. The actual
> >>> memory "movement" is achieved suspending the domain to a ttemporary file 
> >>> and
> >>> resuming it with the new vcpu-affinity
> >> Hmm... this will work and be reliable, but it seems a bit clunky.  Long
> >> term we want to be able to do node migration in the background without
> >> shutting down a VM, right?  If that can be done in the 4.3 timeframe,
> >> then it seems unnecessary to implement something like this.
> > We could do something cleverer for HVM (or hybrid) guests to migrate
> > pages while the guest is live but migrating a page under a PV guest's
> > feet requires quiescing it in the style of a migrate.
> >
> > We could probably manage to make it such that you just need the
> > pause/frob/unpause phases without actually writing RAM to disk, and that
> > infrastructure might be useful for other reasons I suppose. (e.g. I
> > think bad page offlining currently uses a similar save/restore trick)
> Yes, I think doing this chunks at a time is likely to be much 
> preferrable to writing the whole thing to disk and reading it out 
> again.  Doing it "live" may end up with more total overhead, but it 
> wont' require the VM being actually down for the suspend/resume cycle, 
> which on a large guest may be tens of seconds.

Tim also suggested a PV protocol whereby the tools hint to the guest "if
you XENMEM_exchange this MFN you will get back an MFN which is much
better for your performance".

(at least I think that's what Tim suggested, I may have misunderstood)


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