Re: getting processor numbers

Jakub Jelinek
2007-04-04T06:47:39Z

On Tue, Apr 03, 2007 at 07:04:58PM -0700, Paul Jackson wrote:
> Andrew wrote:
> > I'd have thought that in general an application should be querying its
> > present affinity mask - something like sched_getaffinity()? That fixes the
> > CPU hotplug issues too, of course.
>
> The sched_getaffinity call is quick, too, and it nicely reflects any
> cpuset constraints, while still working on kernels which don't have
> CPUSETs configured.
>
> There are really at least four "number of CPUs" answers here, and we
> should be aware of which we are providing. There are, in order of
> decreasing size:
> 1) the size of the kernels cpumask_t (NR_CPUS),
> 2) the maximum number of CPUs that might ever be hotplugged into a
> booted system,
> 3) the current number of CPUs online in that system, and
> 4) the number of CPUs that the current task is allowed to use.
>
> I would suggest that (4) is what we should typically return.
> Certainly it would seem that the use that Ulrich is concerned with,
> by OpenMP, wants (4).
>
> Currently, the sysconf(_SC_NPROCESSORS_CONF) returns (3), by counting
> the CPUs in /proc/stat, which is rather bogus on cpuset, or even
> sched_setaffinity, constrained systems.

OpenMP wants (4) and I'll change it that way.

sysconf(_SC_NPROCESSORS_ONLN) must return (3) (this currently scans /proc/stat)
and
sysconf(_SC_NPROCESSORS_CONF) should IMHO return (2) (this currently
scans /proc/cpuinfo on alpha and sparc{,64} for ((ncpus|CPUs) probed|cpus detected)
and for the rest just returns sysconf(_SC_NPROCESSORS_ONLN)).
Neither of the sysconf returned values should be affected by affinity.

Jakub