Re: [Xen-devel] VM Migration on a NUMA server?

[Dario Faggioli <dario.faggioli@xxxxxxxxxx>]

On Mon, 2015-08-03 at 12:23 +0000, Kun Cheng wrote:

> On Mon, Aug 3, 2015 at 6:10 PM Dario Faggioli
> <dario.faggioli@xxxxxxxxxx> wrote:
> 
>         On Sat, 2015-08-01 at 06:21 +0000, Kun Cheng wrote:
>         > I've looked into those settings and vcpu affinities on Xen
>         wiki page.
>         > However what I'm trying to argue about is memory should be
>         migrated
>         > when vcpus are moved to another node.
>         >
>         I still struggle a bit to understand what you mean. It's
>         exactly what I
>         said (or, at least, what I tried to do) that memory is *not*
>         moved right
>         now.
> OK I get it, memory is not moved right now. 
>
Right.

> What I said was discussing the necessity of doing that.  As according
> to the locality theory (e.g. programs always prone to access the data
> that is recently accessed) and potential memory access delay (memory
> pages are on Node A but vcpus are moved to Node B), then I suppose
> moving memory pages along with the vcpus are necessary. 
>
Ok, then, yes, being able to move memory would be a good thing in my
opinion too.

Of course, given the particular workload we are talking about (virtual
machines), it remains to be seen whether and how much benefit we get
from that, which is really hard to tell without being able to
try/benchmark.

For instance, locality principles applies to single programs, but,
inside a VM, there are multiple programs running, which may limit the
reliability of such principles (or require them to be adapted
accordingly).

> If it can be achieved we should be able to improve the VM execution
> performance, I suppose.(But probably it will introduce overhead as
> moving memory is really really annoying...) .
>
Exactly. So, yes, nce we are able to do it, we'll need to think to what
pages we want to move, and at how much and how often we want to move
them... not just start moving pages around like mad, just because a vcpu
happen to have been scheduled on a "foreign node" for in instance!


> And If my words still confuses you, please feel free to tell me which
> part misleads you :)
>
No, I think it's clear now.

Regards,
Dario

-- 
<<This happens because I choose it to happen!>> (Raistlin Majere)
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Dario Faggioli, Ph.D, http://about.me/dario.faggioli
Senior Software Engineer, Citrix Systems R&D Ltd., Cambridge (UK)

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