OpenMP and shared variable in Fortran which are not shared

Question

I encounter a problem with OpenMP and shared variables I cannot understand. Everything I do is in Fortran 90/95.

Here is my problem: I have a parallel region defined in my main program, with the clause DEFAULT(SHARED), in which I call a subroutine that does some computation. I have a local variable (an array) I allocate and on which I do the computations. I was expecting this array to be shared (because of the DEFAULT(SHARED) clause), but it seems that it is not the case.

Here is an example of what I am trying to do and that reproduce the error I get:

program main
  !$ use OMP_LIB
  implicit none

  integer, parameter :: nx=10, ny=10
  real(8), dimension(:,:), allocatable :: array

  !$OMP PARALLEL DEFAULT(SHARED)
  !$OMP SINGLE
  allocate(array(nx,ny))
  !$OMP END SINGLE

  !$OMP WORKSHARE
  array = 1.
  !$OMP END WORKSHARE

  call compute(array,nx,ny)

  !$OMP SINGLE
  deallocate(array)
  !$OMP END SINGLE
  !$OMP END PARALLEL

contains
  !=============================================================================
  ! SUBROUTINES
  !=============================================================================
  subroutine compute(array, nx, ny)
    !$ use OMP_LIB
    implicit none

    real(8), dimension(nx,ny) :: array
    integer :: nx, ny
    real(8), dimension(:,:), allocatable :: q
    integer :: i, j

    !$OMP SINGLE
    allocate(q(nx,ny))
    !$OMP END SINGLE

    !$OMP WORKSHARE
    q = 0.
    !$OMP END WORKSHARE

    print*, 'q before: ', q(1,1)

    !$OMP DO SCHEDULE(RUNTIME)
    do j = 1, ny
       do i = 1, nx
          if(mod(i,j).eq.0) then
             q(i,j) = array(i,j)*2.
          else
             q(i,j) = array(i,j)*0.5
          endif
       end do
    end do
    !$OMP END DO

    print*, 'q after: ', q(1,1)

    !$OMP SINGLE
    deallocate(q)
    !$OMP END SINGLE

  end subroutine compute
  !=============================================================================
end program main

When I execute it like that, I get a segmentation fault, because the local array q is allocated on one thread but not on the others, and when the others try to access it in memory, it crashes.

If I get rid of the SINGLE region the local array q is allocated (though sometimes it crashes, which make sense, if different threads try to allocate it whereas it is already the case (and actually it puzzles me why it does not crash everytime)) but then it is clearly as if the array q is private (therefore one thread returns me the expected value, whereas the others return me something else).

It really puzzled me why the q array is not shared although I declared my parallel region with the clause DEFAULT(SHARED). And since I am in an orphaned subroutine, I cannot declare explicitely q as shared, since it is known only in the subroutine compute... I am stuck with this problem so far, I could not find a workaround.

Is it normal? Should I expect this behaviour? Is there a workaround? Do I miss something obvious?

Any help would be highly appreciated!

IanH IanH · Accepted Answer · 2013-03-05T18:52:42

q is an entity that is "inside a region but not inside a construct" in terms of OpenMP speak. The subroutine that q is local to is in a procedure that is called during a parallel construct, but q itself does not lexically appear in between the PARALLEL and END PARALLEL directives.

The data sharing rules for such entities in OpenMP then dictate that q is private.

The data sharing clauses such as DEFAULT(SHARED), etc only apply to things that appear in the construct itself (things that lexically appear in between the PARALLEL and END PARALLEL). (They can't apply to things in the region generally - procedures called in the region may have been separately compiled and might be called outside of any parallel constructs.)

OpenMP and shared variable in Fortran which are not shared

2 Answers