Can I deallocate a shared variable by a single thread using OpenMP?

Question

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.

My first thought is why do you want threads_list shared if it is only used by one thread? Why not just have it as private? Keeping your objects private avoids a number of problems that you only get by sharing them. — Ian Bush, Jun 15 '21 at 10:03
@IanBush thanks to your remark, I changed `threads_list` and `threads_list_all` from `shared` variables to `private` ones. I gained about 60 seconds. That's amazing ! — hakim, Jun 15 '21 at 10:48

score 3 · Accepted Answer · answered Jun 15 '21 at 15:34

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 ???

Can I deallocate a shared variable by a single thread using OpenMP?

1 Answers1