CODEWITHSUNDEEP
fortranopenmpgfortran
Clinton Winant
Clinton Winant

Reputation: 734

How to use OpenMP sections to perform independent tasks in parallel

I am trying to understand how to use OpenMP sections. The program listed below is extracted from one of the llnl tutorials, the explanation states: 'Simple program demonstrating that different blocks of work will be done by different threads'.

!!compile with: gfortran -fopenmp -o omp_worksections omp_worksections.f90 !! also need: export OMP_NUM_THREADS=2 (or 3 or 4)

PROGRAM WORKSECTIONS

      INTEGER N, I, NTHREADS, TID, OMP_GET_NUM_THREADS,OMP_GET_THREAD_NUM
      PARAMETER (N=1000)
      REAL A(N), B(N), C(N), D(N)

!     Some initializations
      DO I = 1, N
        A(I) = I * 1.5
        B(I) = I + 22.35
        C(N) = 0.0
        D(N) = 0.0
      ENDDO

!$OMP PARALLEL SHARED(A,B,C,D,NTHREADS), PRIVATE(I,TID)
      TID = OMP_GET_THREAD_NUM()
      IF (TID .EQ. 0) THEN
        NTHREADS = OMP_GET_NUM_THREADS()
        PRINT *, 'Number of threads =', NTHREADS
      END IF
      PRINT *, 'Thread',TID,' starting...'

!$OMP SECTIONS

!$OMP SECTION
      PRINT *, 'Thread',TID,' doing section 1'
      DO I = 1, N
         C(I) = A(I) + B(I)
         if (i.lt.10) then
            WRITE(*,100) TID,I,C(I)
            end if
 100     FORMAT(' Thread',I2,': C(',I2,')=',F8.2)
      ENDDO

!$OMP SECTION
      PRINT *, 'Thread',TID,' doing section 2'
      DO I = 1, N
         if (i.lt.10) then
         D(I) = A(I) * B(I)
         WRITE(*,200) TID,I,D(I)
 200     FORMAT(' Thread',I2,': D(',I2,')=',F8.2)
         endif
      ENDDO


!$OMP END SECTIONS NOWAIT

      PRINT *, 'Thread',TID,' done.'

!$OMP END PARALLEL

END PROGRAM WORKSECTIONS

When I compile and run, the result is:

 Number of threads =           2
 Thread           0  starting...
 Thread           0  doing section 1
Thread 0: C( 1)=   24.85
Thread 0: C( 2)=   27.35
Thread 0: C( 3)=   29.85
Thread 0: C( 4)=   32.35
Thread 0: C( 5)=   34.85
Thread 0: C( 6)=   37.35
Thread 0: C( 7)=   39.85
Thread 0: C( 8)=   42.35
Thread 0: C( 9)=   44.85
 Thread           1  starting...
 Thread           0  doing section 2
Thread 0: D( 1)=   35.03
Thread 0: D( 2)=   73.05
Thread 0: D( 3)=  114.08
Thread 0: D( 4)=  158.10
Thread 0: D( 5)=  205.12
Thread 0: D( 6)=  255.15
Thread 0: D( 7)=  308.18
Thread 0: D( 8)=  364.20
Thread 0: D( 9)=  423.23
 Thread           0  done.
 Thread           1  done.

It seems thread 0 does both sections 1 and 2? I was expecting the prints from either section to be interleaved with one thread doing section 1 and the other section 2.

I have tried removing the NOWAIT clause in the END SECTIONS DIRECTIVE, and removing C,D from the shared clause in the PARALLEL directive, to no avail.

I am obviously missing some central piece of the puzzle?

Upvotes: 1

Views: 659

Answers (1)

Vladimir F Героям слава
Vladimir F Героям слава

Reputation: 59998

It appears that when the OpenMP runtime library is looking for a free thread for the second section, it finds thread 0 free again, because there is too little work to do in the first section. So it assigns the work to thread 0 again.

Try larger n, like 100000:

Number of threads =           2
 Thread           0  starting...
 Thread           0  doing section 1
 Thread 0: C( 1)=   24.85
 Thread 0: C( 2)=   27.35
 Thread 0: C( 3)=   29.85
 Thread 0: C( 4)=   32.35
 Thread 0: C( 5)=   34.85
 Thread           1  starting...
 Thread           1  doing section 2
 Thread 1: D( 1)=   35.03
 Thread 1: D( 2)=   73.05
 Thread 1: D( 3)=  114.08
 Thread 1: D( 4)=  158.10
 Thread 1: D( 5)=  205.12
 Thread 1: D( 6)=  255.15
 Thread 1: D( 7)=  308.18
 Thread 1: D( 8)=  364.20
 Thread 1: D( 9)=  423.23
 Thread 0: C( 6)=   37.35
 Thread 0: C( 7)=   39.85
 Thread 0: C( 8)=   42.35
 Thread 0: C( 9)=   44.85
 Thread           1  done.
 Thread           0  done.

Upvotes: 1

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