parallel do mistake in fortran - fortran

Question

program main
use omp_lib
implicit none
integer :: n=8
integer :: i, j, myid, a(8, 8), b, c(8)
! Generate a 8*8 array A
!$omp parallel default(none), private(i, myid), &
!$omp shared(a, n)
myid = omp_get_thread_num()+1
do i = 1, n
a(i, myid) = i*myid
end do
!$omp end parallel
! Array A
print*, 'Array A is'
do i = 1, n
print*, a(:, i)
end do
! Sum of array A
b = 0
!$omp parallel reduction(+:b), shared(a, n), private(i, myid)
myid = omp_get_thread_num()+1
do i = 1, n
b = b + a(i, myid)
end do
!$omp end parallel
print*, 'Sum of array A by reduction is ', b
b = 0
c = 0
!$omp parallel do
do i = 1, n
do j = 1, n
c(i) = c(i) + a(j, i)
end do
end do
!$omp end parallel do
print*, 'Sum of array A by using parallel do is', sum(c)
!$omp parallel do
do i = 1, n
do j = 1, n
b = b + a(j, i)
end do
end do
!$omp end parallel do
print*, 'Sum of array A by using parallel do in another way is', b
end program main
I wrote a piece of Fortran code above to implement OpenMP to sum up all elements in a 8*8 array in three different ways. First one uses reduction and works. Second, I created a one dimension array with 8 elements. I sum up each column in parallel region and then sum them up. And this works as well. Third one I used an integer to sum up every element in array, and put it in parallel do region. This result is not correct and varies every time. I don't understand why this situation happens. Is because didn't specify public and private or the variable b is overwritten in the procedure?

Answer 1

There is a race condition on b on your third scenario: several threads are reading and writing the same variable without proper synchronization / privatization.
Note that you don't have a race condition in the second scenario: each thread is updating some data (i.e. c(i)) that no one else is accessing.
Finally, some solutions to your last scenario:
Add the reducion(+:b) clause to the pragma
Add a pragma omp atomic directive before the b = b + c(j,i) expression
You can implement a manual privatization