gfortran 4.2 with openMP: why no speedup?

["Anand Patil" <anand dot prabhakar dot patil at gmail dot com>]

Hi all,

I'm trying to figure out why I'm not getting any speedup at all in the
following Fortran code, even though I've set nx and ny so large (1000
and 5000) that the computation takes 14-15s and I'm on an 8-core
machine:

      SUBROUTINE testomp(C,nx,ny)

cf2py threadsafe
cf2py double precision dimension(nx,ny),intent(inplace)::C
cf2py integer intent(hide),depend(C)::nx=shape(C,0)
cf2py integer intent(hide),depend(C)::ny=shape(C,1)

      INTEGER nx,ny,i,j
      DOUBLE PRECISION C(nx,ny)

!$OMP  PARALLEL DO
!$OMP& DEFAULT(SHARED) PRIVATE(i,j)
!$OMP& SCHEDULE(STATIC)
        do i=1,nx
          do j=1,ny
            C(i,j) = dexp(-C(i,j)**2)
          enddo
        enddo
!$OMP  END PARALLEL DO NOWAIT

      return
      END

The 'cf2py's are directives to f2py, the Python-Fortran interface generator.

I'm using gfortran 4.2.1 from Ubuntu Gutsy's apt-get, but because
Python needs to dlopen the shared object I applied the patch from bug
28482 ( http://gcc.gnu.org/bugzilla/show_bug.cgi?id=28482 ),
recompiled just libgomp, and directed the runtime linker to the new
libgomp instead of the old ones.

Any help is greatly appreciated.

Thanks,
Anand Patil