Openmp nested parallelism use available threads - fortran

Question

So, I have this simple Fortran do loop and inside that loop a couple of subroutines are called. I have
Made the do loop parallel with OpenMP, like this
!$omp parallel do
do i=1,n
call a()
call b()
enddo
!$omp end parallel do
Now most of the times the number of iterations in the loop is
less compared to the number of processor/threads available and the subroutines that are called inside the
loop can be called in parallel. So, is there a way to call the subroutines in parallel inside the parallel
do loop ? I have tried with task like this
!$omp parallel do
do i=1,n
!$omp task
call a(i , j )
!$omp end task
!$omp task
call b(i, k)
!$omp end task
!$omp taskwait
enddo
!$omp end parallel do
But this shows some error with segmentation fault. Is there any way to achieve this.
UPDATE:
So, I found out the main reason for the segmentation fault is coming from the fftw library. Lets consider a dummy program
program name
!$use omp_lib
implicit real*8(a-h,p-z)
call system_clock(count_rate=irate)
call system_clock(it1)
!$ call omp_set_nested(.true.)
!$omp parallel do
do i =1,5
call test(i)
print *, i
enddo
!$omp end parallel do
call system_clock(it2)
print *, (it2-it1)/real(irate, kind=8)
end program name
subroutine test(ii)
! just a dummy subroutine for heavy computation
implicit real*8(a-h,p-z)
do j=1,40000
!$omp task
do k=1,40000
x = exp(sqrt(sqrt(2.0d0*ii**3)**2))
enddo
!$omp end task
enddo
end subroutine
This program works exactly what I wants and using the task directives, uses the remaining threads and improves the performance. Now lets consider another dummy program but with fftw, similar to what I'm working.
program name
!$use omp_lib
implicit real*8(a-h,p-z)
integer, parameter :: n=8192*8
complex(kind=8) :: arr(n)
real(kind=8) :: tmp1(n), tmp2(n)
integer(kind=8) :: pF
integer :: i
call system_clock(count_rate=irate)
call dfftw_plan_dft_1d(pF,n,arr,arr,-1,0) ! forward
call system_clock(it1)
!$ call omp_set_nested(.true.)
!$omp parallel do private(arr)
do i =1,5
call random_number(tmp1)
call random_number(tmp2)
arr = cmplx(tmp1, tmp2, kind=8)
call test(pF, arr)
print *, i
enddo
!$omp end parallel do
call system_clock(it2)
print *, (it2-it1)/real(irate, kind=8)
end program name
subroutine test(pF, arr)
implicit real*8(a-h,p-z)
complex(kind=8) :: arr(:)
integer(kind=8) :: pF
do j=1,100
!$omp task private(arr)
do k=1, 100
call dfftw_execute_dft(pF, arr, arr)
enddo
!$omp end task
enddo
end subroutine
Now, this throws the segmentation fault. (NOTE: I have no random numer call in my actual program, they are here just for a dummy purpose). I have checked http://www.fftw.org/fftw3_doc/Thread-safety.html and fftw_execute is thread safe and the program works without the task directives. But with the task it throws error. Anyone knows how to fix this ?

Answer 1

Sigh, yet another example of why !$omp do parallel is a bad idea ... I really do think it is best to clearly separate the thread creation and worksharing phases.
As Vladimir says in the comments you haven't provided nearly enough detail to tell why you are getting a segmentation fault. However you seem to have a few misconceptions about OpenMP which I can try to address.
Firstly a very quick and dirty way to achieve what you want and avoiding any extra OpenMP directives is
!$omp parallel default( none ) private( i ) shared( n ) ! Create threads
!$omp do ! Now share out the work
Do i = 1, 2 * n
If( Mod( i, 2 ) == 1 ) Then
Call a
Else
Call b
End Do
!$omp end do
!$omp end parallel
However if you want to use tasks you're probably not doing it the easiest way if all calls to a and b are completely independent. In that case remember that a new task is created whenever ANY thread hits a !$omp task, and that that task can be executed by any thread, not just the one that created it. Following that logic something like
!$omp parallel default( none ) private( i ) shared( n ) ! Crate the threads
!$omp single
Do i = 1, n
!$omp task
Call a
!$omp end task
!$omp task
call b
!$omp end task
end do
!$omp end single
!$omp end parallel
is what you want - you use one thread to create the list of tasks, and then (or more probably while the list is being created) all the available threads will execute them, each task being taken by the next available thread. Note I have also missed out the taskwait directive as from your description I'm not sure why you think you need it as I can see no need for synchronisation at that point.