Re: [Xen-devel] how to move the memory of a domain from one NUMA node to another?

[Dario Faggioli <dario.faggioli@xxxxxxxxxx>]

On mer, 2013-08-07 at 18:55 +0800, butine@xxxxxxxxxx wrote:
> hello,Dario,
> 
Hi,

> I have been studying in your patch about "memory migration",
>
Oh, you are? Cool... What do you think about'em? :-)

I'm working on a new version of that series from quite a while now, but
I keep being sidetracked by various things. Anyway, as of now, It
shouldn't take much longer to be able to repost something...

> how to recognize the hot memory, 
>
And do you have any idea of that?

The two options I can think of are performance counters (but they're
coarse grained, they're not available on all archs, etc.) and 'hinting
page faults'. That is something Linux does, i.e., it periodically
removes the _PAGE_PRESENT bit from all the pages and, when the page
fault occurs, it make a note of where the page is and from where it is
being accessed (and at that point you can even count how many times,
etc.).

That being said, the point here is, would that be worth? I mean, for
Linux is not a big deal to migrate just one (or a bunch of) page(s) at a
time, for us here, it indeed is quite a big deal (PV guests!!). So I
honestly don't think we can take the same approach and hence, while it
would be useful to have some info about the above, I'm not sure it's
terribly important.

Thoughts?

> and how to gurantee no downtime.
> 
And again, did you reach any conclusion on this? :-)

I do not think that, for PV guest, zero downtime is almost impossible.
Right now I'm doing a very quick suspend-resume alike cycle. We can (in
future) try to turn it into something even more lightweight (some kind
of quiescence state), but you don't change the PTEs under the feet of a
running domain! :-O

> At the same time,In the migration process,the P2M need to be locked?
> 
Well, right now I'm suspending the domain, so I really think it does
not.

Dario

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

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