Re: [Xen-devel] [PATCH 6 of 8 [RFC]] libxc: introduce xc_domain_move_memory

[Dario Faggioli <dario.faggioli@xxxxxxxxxx>]

On mar, 2013-04-09 at 18:43 +0100, Dan Magenheimer wrote:
> > From: Dario Faggioli [mailto:dario.faggioli@xxxxxxxxxx]
> > Subject: [PATCH 6 of 8 [RFC]] libxc: introduce xc_domain_move_memory
> 
> (NUMA discussion...)
> 
Hi Dan,

> > 
> > XXX Open issues are:
> >      - TMEM: how to "move" it?
> 
> (Konrad added to cc list.)
> 
> Tmem memory is, by definition, the lowest priority memory
> for the domain and the hypervisor may already be storing it as
> efficiently as possible (i.e. the page may be deduplicated).
> When it is accessed by the domain (it is never directly
> addressable by a domain, and a hypercall is required
> to access it), an entire page is sequentially copied from
> a physical page in the hypervisor to the domain.  Juergen may
> know otherwise, but I'd guess this inter-node copy would be
> very efficiently pipelined, cache-line by cache-line
> possibly even with hardware pre-fetching.
> 
Ok, thanks for the clarification.

> So the best answer to "how to move it?" may be "don't
> move it at all!".  
>
Ok. I sort of got the feeling that "not touching" would have been TRT
but, again, thanks for making it clear. :-)

> In fact, a good design for a NUMA-aware
> implementation of tmem might intentionally store the data on
> "any node other than the node making this tmem-put hypercall".
> 
Well, we'll get there too, sooner or later. For now, and for the purpose
of this specific work, I'll put things in such a way that they live TMEM
alone.

Regards,
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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