Xen devel - Jan 2010 - Ping: c/s 20526 (tools: avoid cpu over-commitment if numa=on)

Andre,

I''m afraid this change isn''t really correct:
>+                cores_per_node = info[''nr_cpus''] /
info[''nr_nodes'']
>+                nodes_required = (self.info[''VCPUs_max'']
+ cores_per_node - 1) / cores_per_node
Simply using cores_per_node (as calculated here) as a divisor is bound
to cause division-by-zero issues, namely when limiting the number of
CPUs on the Xen command line (maxcpus=). I''m not sure though, what
a reasonable solution to this might look like, since cores-per-node is
a meaningless thing in an artificial setup like this, and may also be
meaningless in asymmetric configurations. So perhaps we really need
to iterate over nodes while summing up the number of CPUs they
have until the number of needed vCPU-s was reached.

Jan



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On 13/01/2010 08:15, "Jan Beulich" <JBeulich@novell.com> wrote:
> Andre,
> 
> I''m afraid this change isn''t really correct:
> 
>> +                cores_per_node = info[''nr_cpus''] /
info[''nr_nodes'']
>> +                nodes_required =
(self.info[''VCPUs_max''] + cores_per_node -
>> 1) / cores_per_node
> 
> Simply using cores_per_node (as calculated here) as a divisor is bound
> to cause division-by-zero issues, namely when limiting the number of
> CPUs on the Xen command line (maxcpus=). I''m not sure though, what
> a reasonable solution to this might look like, since cores-per-node is
> a meaningless thing in an artificial setup like this, and may also be
> meaningless in asymmetric configurations. So perhaps we really need
> to iterate over nodes while summing up the number of CPUs they
> have until the number of needed vCPU-s was reached.
Yes, please!

 K.



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Jan Beulich wrote:
> Andre,
> 
> I''m afraid this change isn''t really correct:
> 
>> +                cores_per_node = info[''nr_cpus''] /
info[''nr_nodes'']
>> +                nodes_required =
(self.info[''VCPUs_max''] + cores_per_node - 1) / cores_per_node
> 
> Simply using cores_per_node (as calculated here) as a divisor is bound
> to cause division-by-zero issues, namely when limiting the number of
> CPUs on the Xen command line (maxcpus=). I''m not sure though, what
> a reasonable solution to this might look like, since cores-per-node is
> a meaningless thing in an artificial setup like this, and may also be
> meaningless in asymmetric configurations. So perhaps we really need
> to iterate over nodes while summing up the number of CPUs they
> have until the number of needed vCPU-s was reached.
Thanks for the heads-up. I have created a better patch basically doing 
what you described, actually it is even more elegant than the original 
version. I am about to test it and will send it out as soon as the 
testing is finished.

Regards,
Andre.

-- 
Andre Przywara
AMD-Operating System Research Center (OSRC), Dresden, Germany
Tel: +49 351 448 3567 12
----to satisfy European Law for business letters:
Advanced Micro Devices GmbH
Karl-Hammerschmidt-Str. 34, 85609 Dornach b. Muenchen
Geschaeftsfuehrer: Andrew Bowd; Thomas M. McCoy; Giuliano Meroni
Sitz: Dornach, Gemeinde Aschheim, Landkreis Muenchen
Registergericht Muenchen, HRB Nr. 43632


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Hi,

as Jan Beulich pointed out:
> I''m afraid this change isn''t really correct:
> 
>> +                cores_per_node = info[''nr_cpus''] /
info[''nr_nodes'']
>> +                nodes_required =
(self.info[''VCPUs_max''] + cores_per_node - 1) / cores_per_node
> 
> Simply using cores_per_node (as calculated here) as a divisor is bound
> to cause division-by-zero issues, namely when limiting the number of
> CPUs on the Xen command line (maxcpus=).
Actually Jan''s proposed method of getting additional nodes is more 
elegant, so I implemented: enumerating the best nodes and adding CPU 
affinity until all VCPUs can be backed by at least on physical core. 
This should fix problems with asymmetric NUMA configurations and cropped 
number of CPUs in Xen.

Regards,
Andre.

Signed-off-by: Andre Przywara <andre.przywara@amd.com>

-- 
Andre Przywara
AMD-Operating System Research Center (OSRC), Dresden, Germany
Tel: +49 351 488-3567-12
----to satisfy European Law for business letters:
Advanced Micro Devices GmbH
Karl-Hammerschmidt-Str. 34, 85609 Dornach b. Muenchen
Geschaeftsfuehrer: Andrew Bowd; Thomas M. McCoy; Giuliano Meroni
Sitz: Dornach, Gemeinde Aschheim, Landkreis Muenchen
Registergericht Muenchen, HRB Nr. 43632


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