Re: [Xen-devel] [PATCH v1][RFC] drivers/xen, balloon driver numasupport in kernel

[Dario Faggioli]

On lun, 2013-08-12 at 19:44 +0100, David Vrabel wrote:
> On 12/08/13 15:13, Yechen Li wrote:
> >   This small patch adds numa support for balloon driver. Kernel version: 3.11-rc5
> >   It's just a RFC version, since I'm waiting for the interface of numa topology.
> >   The balloon driver will read arguments from xenstore: /local/domain/(id)/memory
> > /target_nid, and settle the memory increase/decrease operation on specified
> > p-nodeID.
> 
> Its is difficult to review an ABI change without any documentation for
> the new ABI.
> 
Indeed.

> I would also like to see a design document explaining the overall
> approach planned to be used here.  It's not clear why explicitly
> specifying nodes is preferable to (e.g.) the guest releasing/populating
> evenly across all its nodes (this would certainly be better for the guest).
> 
I see what you mean. Personally, I think they're different things. There
might be the need, from the host system administrator, to make as much
room as possible on one (or perhaps a few) nodes, in which case, the
possibility of specifying that explicitly would be a plus.

That would allow --if used wisely, I agree with you on this-- for better
resource utilization, in the long run.

In absence of this information, it is probably true that the guest would
benefit from a more even approach. What we want to achieve here,
however, is as follows: suppose that a virtual NUMA enabled guest (i.e.,
a guest with a virtual NUMA topology), has guest page X, which is on
virtual node g1 in the guest itself, backed by a frame from host node
h0. Well, we really would like to try having page X always backed by a
frame on host node h1, even after ballooning down and up.

> It seems like unless this is used carefully, all VMs will end up with
> suboptimal memory layouts as they are repeatedly balloon up and down to
> satisfy the whims of the latest VM being started etc.
> 
I'm not sure I see entirely what you mean, but for sure I repeat that I
agree that more information about the design and intended usage patterns
are needed... Let's see whether Yechen is up for providing that. :-)

Thanks for having a look anyway,
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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