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 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/