Re: [Xen-users] best practice NUMA config for dom0 ?

[Dario Faggioli <dario.faggioli@xxxxxxxxxx>]

On Wed, 2016-05-18 at 21:18 +0200, Håkon Alstadheim wrote:
> Den 18. mai 2016 18:23, skrev Dario Faggioli:
> > Also, it would be helpful if also you tell us the characteristics
> > of
> > both the host and the guests, such as:
> >  - number of pCPUs
> >  - number of NUMA nodes
> >  - amount or RAM per node
> >  - amount of free RAM in each node, as reported by `xl info -n'
> Ram-amount might have played a part. If I remember correctly I had
> one
> rather large VM running in addition to what I usually do, when I
> noticed
> the error. 
>
This is likely to be the cause of it, then.

> It happened intermittently seven times during two days about
> a week ago, with two different vms. Will investigate further.
> > 
> >  - number of vCPUs of the guest
> >  - amount of RAM of the guest
> I seem to be unable to reproduce the errors at present. 
>
Hehe, I see. :-)

> Since I reported
> the errors I have enabled some basic NUMA awareness in dom0, but that
> should not have any bearing on Xen behaviour, and no benefit for
> dom0,
> if I understand you correctly.
> 
No, I'm absolutely sure that, no matter what you have added in dom0,
that is not what is influencing things an making the error disappear.

In fact, I'd bet that your dom0 --if queried with specific tools, like,
e.g., numactl-- reports no NUMA-ness at all, doesn't it?

As per the automatic placement error, as said above, typical causes
would be not enough free memory in any of the host NUMA node to
accommodate the guest, or too many vCPUs in the guest (more than the
host has in one or a certain set of nodes).

Anyway, after having created the guest, check the output of:

# xl info -n
# xl list -n
# xl vcpu-list

These would give you some indication of what is going on, from a NUMA
perspective.

It's a very limited interface, I know... we're working to add more
stuff.

> I will provide better info if I am able to reproduce at a later date.
>
Sure, any time. :-)

> For now I'm satisfied that this was not a misunderstanding on my part
> regarding NUMA config.
> 
Right.

Best 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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