https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108494
Bug ID: 108494
Summary: Slow thread creation with nested loops in GFortran
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: libgomp
Assignee: unassigned at gcc dot gnu.org
Reporter: dewhu...@mpi-halle.mpg.de
CC: jakub at gcc dot gnu.org
Target Milestone: ---
This is an issue with very slow thread creation for nested loops in code
compiled with GFortran, however I suspect it may be due to the libgomp library.
Here is a simple example the problem:
program test
implicit none
integer l
!$OMP PARALLEL DO &
!$OMP NUM_THREADS(1)
do l=1,1000
call foo
end do
!$OMP END PARALLEL DO
end program
subroutine foo
implicit none
integer, parameter :: l=200,m=100,n=10
! number of threads
integer, parameter :: nthd=10
integer i,j
! automatic arrays
real(8) a(n,l),b(n,m),x(m)
a(:,:)=2.d0
b(:,:)=3.d0
do i=1,l
!$OMP PARALLEL DO DEFAULT(SHARED) &
!$OMP NUM_THREADS(nthd)
do j=1,m
x(j)=dot_product(a(:,i),b(:,j))
end do
!$OMP END PARALLEL DO
end do
end subroutine
The wall-clock time is about 0.5 seconds when compiled with Intel or PGI
Fortran. However, for GFortran compiled with
gfortran -O3 -fopenmp test.f90
and OMP_NESTED set to true, the wall-clock time is about 70 seconds, or about
140 times slower. (The ‘dot_product’ can be removed from the loop – all the
time is taken with thread creation).
This only affects nested loops; if the OMP directives are removed from the loop
in the program part in the code above then GFortran is as fast as the other
compilers. I’ve tried several different versions of GFortran (from 7.5.0 to
12.1.0) on different Linux machines and it’s slow on all of them.
It may problem with libgomp. If I substitute the libgomp library for that
provided with the NVIDIA compiler (on our machine this is in the directory
nvhpcsdk/22.11/Linux_x86_64/22.11/compilers/lib/libgomp.so.1) then it’s as fast
as the others.
This has been reproduced by others and also in Windows, see here:
https://fortran-lang.discourse.group/t/slow-thread-creation-with-nested-loops-in-gfortran/5062
