Re: [libvirt] Problem of host CPU topology parsing
On Fri, May 11, 2012 at 10:53:12 +0100, Daniel P. Berrange wrote:
On Fri, May 11, 2012 at 05:42:34PM +0800, Osier Yang wrote:
> On 2012年05月11日 17:01, Jiri Denemark wrote:
> >On Fri, May 11, 2012 at 10:47:06 +0200, Michal Privoznik wrote:
> >>On 11.05.2012 10:40, Osier Yang wrote:
> >>> /* nodeinfo->sockets is supposed to be a number of sockets per
NUMA
> >>>node,
> >>> * however if NUMA nodes are not composed of whole sockets, we just
lie
> >>> * about the number of NUMA nodes and force apps to check
> >>>capabilities XML
> >>> * for the actual NUMA topology.
> >>> */
> >>> if (nodeinfo->sockets % nodeinfo->nodes == 0)
> >>> nodeinfo->sockets /= nodeinfo->nodes;
> >>> else
> >>> nodeinfo->nodes = 1;
> >>>
> >>>Jirka said this was for a fix, but I don't quite understand it,
> >>>what does the "nodeinfo.nodes" mean actually? Shouldn't
it
> >>>be 8 (for the 48 CPUs machine) instead? But then we will be
> >>>wrong again with using VIR_NODEINFO_MAXCPUS.
> >>
> >>Why do you think it will be wrong? My understanding is that
> >>VIR_NODEINFO_MAXCPUS just tell the max number of possible cpus not the
> >>actual. So if it's over 48 we are safe.
> >
> >Not really, the macro should count exactly the number of CPUs available to
> >host, otherwise lots of other issues (incl. backward compatibility) appear. It
> >is just a badly named macro that should never exist but we can't do
anything
> >with it since it is our public API.
> >
> >>Btw: the code above seems like a hack to me.
> >
> >Yes, it is a hack but it's unfortunately required because we can't
change the
> >macro.
> >
> >Anyway, I agree with Daniel that the bug most likely lies somewhere in the
> >code that populates nodeinfo structure.
> >
> >Jirka
>
> In /proc/cpuinfo:
>
> <snip>
> cpu cores : 12
> </snip>
>
> However, there are only 6 core IDs, as showed in
> http://fpaste.org/mtoA/. And we parse the core_id
> file of each CPU as:
>
> core = parse_core(cpu);
> if (!CPU_ISSET(core, &core_mask)) {
> CPU_SET(core, &core_mask);
> nodeinfo->cores++;
> }
>
> and thus get only 6 cores. Don't known how 12 in /proc/cpuinfo
> is figured out. But could it be a clue?
Ahhh. The AMD 12 "core" CPUs are in fact a pair of 6 core CPUs
with 2 NUMA nodes in the CPU itself.
Oh, so the problem is that two 6-core CPUs share the same socket and thus have
the same physical ID. So it's either 8 6-core CPUs or 4 12-core CPUs. Not
sure which one is better to present. The first one is the real thing and the
second one is how AMD presents the reality :-) Anyway, we should do something
with
/* Parse core */
core = parse_core(cpu);
if (!CPU_ISSET(core, &core_mask)) {
CPU_SET(core, &core_mask);
nodeinfo->cores++;
}
/* Parse socket */
sock = parse_socket(cpu);
if (!CPU_ISSET(sock, &socket_mask)) {
CPU_SET(sock, &socket_mask);
nodeinfo->sockets++;
}
which just ignores duplicate physical/core IDs. I feel like this was added
there for some reason, though...
Jirka