10:11 a.m.
On Mon, Mar 26, 2012 at 05:08:05PM +0200, Jiri Denemark wrote:
On Mon, Mar 26, 2012 at 15:42:58 +0100, Daniel P. Berrange wrote:
> On my x86_64 host I have a pair of Quad core CPUs, each in a separate
> NUMA node. The virsh capabilities
> topology data reports this:
>
> # virsh capabilities | xmllint --xpath /capabilities/host/cpu -
> <cpu>
> <arch>x86_64</arch>
> <model>Opteron_G3</model>
> <vendor>AMD</vendor>
> <topology sockets="1" cores="4"
threads="1"/>
> <feature name="osvw"/>
> <feature name="3dnowprefetch"/>
> <feature name="cr8legacy"/>
> <feature name="extapic"/>
> <feature name="cmp_legacy"/>
> <feature name="3dnow"/>
> <feature name="3dnowext"/>
> <feature name="pdpe1gb"/>
> <feature name="fxsr_opt"/>
> <feature name="mmxext"/>
> <feature name="ht"/>
> <feature name="vme"/>
> </cpu>
> # virsh capabilities | xmllint --xpath /capabilities/host/topology -
> <topology>
> <cells num="2">
> <cell id="0">
> <cpus num="4">
> <cpu id="0"/>
> <cpu id="1"/>
> <cpu id="2"/>
> <cpu id="3"/>
> </cpus>
> </cell>
> <cell id="1">
> <cpus num="4">
> <cpu id="4"/>
> <cpu id="5"/>
> <cpu id="6"/>
> <cpu id="7"/>
> </cpus>
> </cell>
> </cells>
> </topology>
>
> Note, it is reporting sockets=1, because sockets is the number of sockets
> *per* NUMA node.
>
>
> Now I try to figure the guest to match the host using:
>
> <cpu>
> <topology sockets='1' cores='4' threads='1'/>
> <numa>
> <cell cpus='0-3' memory='512000'/>
> <cell cpus='4-7' memory='512000'/>
> </numa>
> </cpu>
>
> And I get:
>
> error: Maximum CPUs greater than topology limit
>
> So, the XML checker is mistaking 'sockets' as the total number of sockets,
> rather than the per-node socket count. We need to fix this bogus check
I guess what we actually want to do is to report total number of sockets in
host cpu xml. Sockets per NUMA node has been proven to be a bad decision and
we should not let it infect other areas.
No, we can't change that - we explicitly fixed that a while back
because it breaks the VIR_NODEINFO_MAXCPUS macro to do that.
commit ac9dd4a676f21b5e3ca6dbe0526f2a6709072beb
Author: Jiri Denemark <jdenemar(a)redhat.com>
Date: Wed Nov 24 11:25:19 2010 +0100
Fix host CPU counting on unusual NUMA topologies
The nodeinfo structure includes
nodes : the number of NUMA cell, 1 for uniform mem access
sockets : number of CPU socket per node
cores : number of core per socket
threads : number of threads per core
which does not work well for NUMA topologies where each node does not
consist of integral number of CPU sockets.
We also have VIR_NODEINFO_MAXCPUS macro in public libvirt.h which
computes maximum number of CPUs as (nodes * sockets * cores * threads).
As a result, we can't just change sockets to report total number of
sockets instead of sockets per node. This would probably be the easiest
since I doubt anyone is using the field directly. But because of the
macro, some apps might be using sockets indirectly.
This patch leaves sockets to be the number of CPU sockets per node (and
fixes qemu driver to comply with this) on machines where sockets can be
divided by nodes. If we can't divide sockets by nodes, we behave as if
there was just one NUMA node containing all sockets. Apps interested in
NUMA should consult capabilities XML, which is what they probably do
anyway.
This way, the only case in which apps that care about NUMA may break is
on machines with funky NUMA topology. And there is a chance libvirt
wasn't able to start any guests on those machines anyway (although it
depends on the topology, total number of CPUs and kernel version).
Nothing changes at all for apps that don't care about NUMA.
Daniel
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