I am using OpenMP in order to parallelize a code. Here is the most important part of the code according to the question that I will ask:
!$OMP PARALLEL PRIVATE(num_thread) &
!$OMP SHARED(tasklist_GRAD,threads_list,threads_list_all,tasks_ready_master) &
!$OMP SHARED(threads_list_part1,nthreads)
num_thread=OMP_GET_THREAD_NUM() ! le rang du thread
nthreads=OMP_GET_NUM_THREADS() ! le nombre de threads
!Thread Application Master (numero 1)
if (num_thread==1) then
do ff=1,3 ! 3 tâches
if (associated(tasklist_GRAD(ff)%f_ptr) .eqv. .true. ) then ! Si tâche attribuée
tasks_ready_master(ff) = tasklist_GRAD(ff) ! égalité de pointeurs
tasks_ready_master(ff)%state=STATE_READY
end if
end do
end if
!$OMP BARRIER
!Thread Master (numero 0)
if (num_thread==0) then
allocate(threads_list(nthreads-2)) ! liste des threads workers
do ff=1,nthreads-2
threads_list(ff)=ff+1 ! 2,3,..,nombre de threads-2
end do
do ff=1,3,nthreads-2
if (tasks_ready_master(ff)%state==STATE_READY) then
threads_list_all(ff:ff+nthreads-3)=threads_list(:)
end if
end do
threads_list_part1=threads_list_all(1:3) ! 3 tâches
deallocate(threads_list)
end if
!$OMP BARRIER
As you can see, threads_list is a shared variable. My question is very simple.
Do I have the right to deallocate a shared variable by 1 single thread or should I retire the if (num_thread==0) then in order to get this done by all the threads ?
I'm asking this since I got an error related to memory leaks.
To answer the question as there is only one instantiation of the shared variable, it need be deallocated only once - but it doesn't matter by which thread:
ijb@ijb-Latitude-5410:~/work/stack$ cat omp.f90
Program omp_alloc
Use omp_lib, Only : omp_get_num_threads, omp_get_thread_num
Implicit None
Integer, Dimension( : ), Allocatable :: a
Integer :: ith
Integer :: nth
Integer :: i
Allocate( a( 1:4 ) )
a = [ ( i, i = 1, 4 ) ]
!$omp parallel default( none ) shared( a ) private( ith, nth )
nth = omp_get_num_threads()
ith = omp_get_thread_num ()
Write( *, * ) ith, nth, 'a = ', a
! Barrier to make sure a is printed by all threads before deallocation
!$omp barrier
!$omp single
Write( *, * ) 'Thread ', ith, ' Deallocating'
Deallocate( a )
!$omp end single
!$omp end parallel
Write( *, * ) Allocated( a )
End Program omp_alloc
ijb@ijb-Latitude-5410:~/work/stack$ gfortran-11 --version
GNU Fortran (GCC) 11.1.0
Copyright © 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.
ijb@ijb-Latitude-5410:~/work/stack$ gfortran-11 -Wall -Wextra -fcheck=all -O -g -std=f2018 -fopenmp omp.f90
ijb@ijb-Latitude-5410:~/work/stack$ export OMP_NUM_THREADS=4
ijb@ijb-Latitude-5410:~/work/stack$ ./a.out
0 4 a = 1 2 3 4
2 4 a = 1 2 3 4
3 4 a = 1 2 3 4
1 4 a = 1 2 3 4
Thread 3 Deallocating
F
ijb@ijb-Latitude-5410:~/work/stack$ ./a.out
0 4 a = 1 2 3 4
1 4 a = 1 2 3 4
3 4 a = 1 2 3 4
2 4 a = 1 2 3 4
Thread 0 Deallocating
F
ijb@ijb-Latitude-5410:~/work/stack$ ./a.out
0 4 a = 1 2 3 4
3 4 a = 1 2 3 4
1 4 a = 1 2 3 4
2 4 a = 1 2 3 4
Thread 0 Deallocating
F
ijb@ijb-Latitude-5410:~/work/stack$ ./a.out
0 4 a = 1 2 3 4
1 4 a = 1 2 3 4
3 4 a = 1 2 3 4
2 4 a = 1 2 3 4
Thread 0 Deallocating
F
ijb@ijb-Latitude-5410:~/work/stack$ ./a.out
0 4 a = 1 2 3 4
3 4 a = 1 2 3 4
2 4 a = 1 2 3 4
1 4 a = 1 2 3 4
Thread 0 Deallocating
F
ijb@ijb-Latitude-5410:~/work/stack$ ./a.out
0 4 a = 1 2 3 4
1 4 a = 1 2 3 4
3 4 a = 1 2 3 4
2 4 a = 1 2 3 4
Thread 0 Deallocating
F
ijb@ijb-Latitude-5410:~/work/stack$ ./a.out
0 4 a = 1 2 3 4
3 4 a = 1 2 3 4
1 4 a = 1 2 3 4
2 4 a = 1 2 3 4
Thread 0 Deallocating
F
ijb@ijb-Latitude-5410:~/work/stack$ ./a.out
0 4 a = 1 2 3 4
3 4 a = 1 2 3 4
1 4 a = 1 2 3 4
2 4 a = 1 2 3 4
Thread 2 Deallocating
F
If you try to deallocate by multiple threads you get undefined behaviour. On gfortran at least his manifests it self by looking like a race condition on the test on whether the array is allocated when you try to deallocate it:
ijb@ijb-Latitude-5410:~/work/stack$ cat omp.f90
Program omp_alloc
Use omp_lib, Only : omp_get_num_threads, omp_get_thread_num
Implicit None
Integer, Dimension( : ), Allocatable :: a
Integer :: ith
Integer :: nth
Integer :: i
Allocate( a( 1:4 ) )
a = [ ( i, i = 1, 4 ) ]
!$omp parallel default( none ) shared( a ) private( ith, nth )
nth = omp_get_num_threads()
ith = omp_get_thread_num ()
Write( *, * ) ith, nth, 'a = ', a
!$omp barrier
Deallocate( a )
!$omp end parallel
Write( *, * ) Allocated( a )
End Program omp_alloc
ijb@ijb-Latitude-5410:~/work/stack$ gfortran-11 -Wall -Wextra -fcheck=all -O -g -std=f2018 -fopenmp omp.f90
ijb@ijb-Latitude-5410:~/work/stack$ export OMP_NUM_THREADS=8
ijb@ijb-Latitude-5410:~/work/stack$ ./a.out
0 8 a = 1 2 3 4
7 8 a = 1 2 3 4
2 8 a = 1 2 3 4
4 8 a = 1 2 3 4
1 8 a = 1 2 3 4
6 8 a = 1 2 3 4
3 8 a = 1 2 3 4
5 8 a = 1 2 3 4
F
ijb@ijb-Latitude-5410:~/work/stack$ ./a.out
6 8 a = 1 2 3 4
0 8 a = 1 2 3 4
2 8 a = 1 2 3 4
5 8 a = 1 2 3 4
4 8 a = 1 2 3 4
3 8 a = 1 2 3 4
7 8 a = 1 2 3 4
1 8 a = 1 2 3 4
At line 21 of file omp.f90
Fortran runtime error: Attempt to DEALLOCATE unallocated 'a'
Error termination. Backtrace:
#0 0x7fe058962d01 in ???
#1 0x7fe058963849 in ???
#2 0x7fe058963ec6 in ???
#3 0x4012fa in MAIN__._omp_fn.0
at /home/ijb/work/stack/omp.f90:21
#4 0x7fe0587cb77d in ???
#5 0x7fe058732608 in start_thread
at /build/glibc-YbNSs7/glibc-2.31/nptl/pthread_create.c:477
#6 0x7fe058657292 in ???
#7 0xffffffffffffffff in ???
ijb@ijb-Latitude-5410:~/work/stack$ ./a.out
0 8 a = 1 2 3 4
3 8 a = 1 2 3 4
1 8 a = 1 2 3 4
6 8 a = 1 2 3 4
7 8 a = 1 2 3 4
2 8 a = 1 2 3 4
4 8 a = 1 2 3 4
5 8 a = 1 2 3 4
F
ijb@ijb-Latitude-5410:~/work/stack$ ./a.out
6 8 a = 1 2 3 4
0 8 a = 1 2 3 4
2 8 a = 1 2 3 4
1 8 a = 1 2 3 4
5 8 a = 1 2 3 4
4 8 a = 1 2 3 4
7 8 a = 1 2 3 4
3 8 a = 1 2 3 4
F
ijb@ijb-Latitude-5410:~/work/stack$ ./a.out
0 8 a = 1 2 3 4
4 8 a = 1 2 3 4
2 8 a = 1 2 3 4
1 8 a = 1 2 3 4
3 8 a = 1 2 3 4
5 8 a = 1 2 3 4
7 8 a = 1 2 3 4
6 8 a = 1 2 3 4
F
ijb@ijb-Latitude-5410:~/work/stack$ ./a.out
0 8 a = 1 2 3 4
3 8 a = 1 2 3 4
7 8 a = 1 2 3 4
2 8 a = 1 2 3 4
4 8 a = 1 2 3 4
6 8 a = 1 2 3 4
5 8 a = 1 2 3 4
1 8 a = 1 2 3 4
F
ijb@ijb-Latitude-5410:~/work/stack$ ./a.out
0 8 a = 1 2 3 4
1 8 a = 1 2 3 4
3 8 a = 1 2 3 4
2 8 a = 1 2 3 4
4 8 a = 1 2 3 4
7 8 a = 1 2 3 4
5 8 a = 1 2 3 4
6 8 a = 1 2 3 4
At line 21 of file omp.f90
Fortran runtime error: Attempt to DEALLOCATE unallocated 'a'
Error termination. Backtrace:
#0 0x7fef41e85d01 in ???
#1 0x7fef41e86849 in ???
#2 0x7fef41e86ec6 in ???
#3 0x4012fa in MAIN__._omp_fn.0
at /home/ijb/work/stack/omp.f90:21
#4 0x7fef41cee77d in ???
#5 0x7fef41c55608 in start_thread
at /build/glibc-YbNSs7/glibc-2.31/nptl/pthread_create.c:477
#6 0x7fef41b7a292 in ???
#7 0xffffffffffffffff in ???