Re: Spurious "NUMA placement failed, performance might be affected" message on ARM

[Dario Faggioli <dario.faggioli@citrix.com>]

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On ven, 2014-04-25 at 11:37 +0100, Ian Campbell wrote:
> Hi Dario,
> 
> When starting a guest on ARM I'm seeing (with no additional verbosity):
>         libxl: notice: libxl_numa.c:494:libxl__get_numa_candidate: NUMA placement failed, performance might be affected
> 
> Which is a bit strong for a non-NUMA system.
> 
Indeed.

> Is there some hypercall we need to stub out or return -ENOSYS from to
> cause this function to decide that this is not a NUMA system?
> 
> Does the same message occur on non-NUMA x86 systems?
> 
The message is printed inside libxl__get_numa_candidate() if no suitable
placement candidate is found. It does not happen on x86 non-NUMA boxes
as what happens there is that there is only 1 node, so the set of
possible combinations of nodes is made up of only one element, which is
deemed to be the best possible solution very quickly.

While I wonder why that does not happen on ARM, a sensible solution
would be to bail earlier, if we find only one NUMA node exist, for
whatever arch. Would that be ok? If yes, I can arrange a patch pretty
easily, I think.

For figuring out why the different behavior... Do you have the output of
`xl info -n' on that box handy, by any chance?

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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