Available runtime processors is not really number of available processors #3963
Description
This function returns the number of available processors by reading from _SC_NPROCESSORS_ONLN, which utilizes get_nprocs(). The get_nprocs() function operates similarly to lscpu and does not indicate the number of processors a program can access.
Here is a code to show the difference:
#include <stdio.h>
#include <stdlib.h>
#include <sys/sysinfo.h>
#include <omp.h>
int main()
{
printf("This system has %d processors configured and "
"%d processors available.\n",
get_nprocs_conf(), get_nprocs());
printf("and in OMP: %d\n", omp_get_max_threads());
exit(EXIT_SUCCESS);
}The output for different numbers of available threads (using taskset) is as follows:
➜ g++ a.cpp -fopenmp
➜ taskset -c 0-0 ./a.out
This system has 8 processors configured and 8 processors available.
and in OMP: 1
➜ taskset -c 0-1 ./a.out
This system has 8 processors configured and 8 processors available.
and in OMP: 2
➜ taskset -c 0-4 ./a.out
This system has 8 processors configured and 8 processors available.
and in OMP: 5
➜ taskset -c 0-100 ./a.out
This system has 8 processors configured and 8 processors available.
and in OMP: 8
This may cause ambiguity in what available processors mean.