https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108336
Bug ID: 108336
Summary: Repeatable random_numbers with openmp
Product: gcc
Version: 11.3.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: fortran
Assignee: unassigned at gcc dot gnu.org
Reporter: gareth.davies.ga.code at gmail dot com
Target Milestone: ---
The aim is to get repeatable random numbers with openmp. This report was
derived from a discussion including input from gfortran developer Steve Kargl:
https://fortran-lang.discourse.group/t/replicable-monte-carlo-simulation-with-openmp/4972/19
AFAIK the following program should give repeatable random numbers using openmp.
But it doesn't - there are occasional changes in results, suggestive of a race
condition.
$ gfortran --version
GNU Fortran (Ubuntu 11.3.0-1ubuntu1~22.04) 11.3.0
Copyright (C) 2021 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
$ cat random_number7.f90
program main
use omp_lib
implicit none
integer :: n1, i, n, nt
real :: x1, x2
real, allocatable :: threadval(:)
n1 = 2
! Initialize the master state to a repeatable sequence.
call random_init(.true., .false.)
! Store final random number on each thread
!$omp parallel
nt = omp_get_num_threads()
!$omp end parallel
allocate(threadval(nt))
threadval = -1.0
!$omp parallel private(x1, x2, i), shared(n1, threadval)
! Trigger non-master threads to get their unique seed
if(omp_get_thread_num() /= 0) call random_number(x1)
!$omp barrier
! Get some random numbers -- almost repeatable (but not)
!$omp do
do i = 1, n1
call random_number(x1)
call random_number(x2)
end do
!$omp end do
! Store final RN on each thread
threadval(omp_get_thread_num()+1) = x2
!$omp end parallel
print*, minval(threadval), maxval(threadval)
end program
$ gfortran -fopenmp random_number7.f90 -o random_number7
$ for i in {1..10}; do OMP_NUM_THREADS=6 ./random_number7; done
0.191325366 0.876643896
0.191325366 0.480524838
0.191325366 0.876643896
0.191325366 0.804360509
0.191325366 0.804360509
0.191325366 0.480524838
0.191325366 0.670394361
0.191325366 0.804360509
0.157898605 0.191325366
0.191325366 0.670394361
