https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86146
Bug ID: 86146
Summary: OpenMP not seeing more than 64 cores on Windows
Product: gcc
Version: 8.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: libgomp
Assignee: unassigned at gcc dot gnu.org
Reporter: karolina1980 at realemail dot net
CC: jakub at gcc dot gnu.org
Target Milestone: ---
Created attachment 44273
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=44273&action=edit
Support Systems That Have More Than 64 Processors
Libgomp can't see more than 64 processors on Windows. Sample program:
#include <stdio.h>
#include <omp.h>
int main(void) {
printf("number of CPUs: %d\n", omp_get_num_procs());
return 0;
}
command line:
gcc -fopenmp -test_cpu.c -o test_cpu
This is relevant for dual CPU systems which have more than 64 logical
processors total. I've looked into libgomp implementation, proc.c file contains
this:
static unsigned int
count_avail_process_cpus ()
{
DWORD_PTR process_cpus;
DWORD_PTR system_cpus;
if (GetProcessAffinityMask (GetCurrentProcess (),
&process_cpus, &system_cpus))
{
unsigned int count;
for (count = 0; process_cpus != 0; process_cpus >>= 1)
if (process_cpus & 1)
count++;
return count;
}
return 1;
}
This is sadly incorrect way to count CPUs on Windows. As GetProcessAffinityMask
gets mask only for current processor group not all CPUs on the system.
See:
https://blogs.technet.microsoft.com/hardtofind/2018/01/29/windows-server-with-more-than-64-logical-processors/
https://msdn.microsoft.com/en-us/library/dd405527(VS.85).aspx
(also see attachment which contains explanation from Microsoft)
Clang 5.0.2 does counting correctly. It's done in kmp_affinity.cpp in Clang
source. Function:
void __kmp_affinity_entire_machine_mask(kmp_affin_mask_t *mask)
Sadly it seems omp is no longer included in Clang for Windows in 6.0.
I have no experience working on gcc or openmp. I am unable to diagnose how many
other functions need to be re-written to support more than 64 CPUs.
