[Beowulf] OpenMP on AMD dual core processors

rssr at lncc.br rssr at lncc.br
Fri Nov 21 10:05:25 EST 2008



Hi All

I thing the problem could be the  convergence
"loop" test and the criation of threads

10 converged = 1
!$OMP PARALLEL
!$OMP DO
.....
!$OMP ENDDO
!$OMP END PARALLEL
if(converged.eq.0) then
  
     goto 10
endif

Each time you "goto 10" 
the compiler "create" and "initialize" the threads
and this is time comsuming.
try to change the convergence test to a
reduce operation this will 
take time but not some much as !$OMP
PARALLEL 
I hope its help

Renato Silva


> Hi All,
> 
> I'm getting to the end of a semester of computational
physics at my
> institution, and thought it would be fin to close
the semester with a
> discussion of parallel programming. 
Initially, I was simply planning to
> discuss MPI, but while
reading through the gfortran man page I realized
> that
>
gcc now supports OpenMP directives.
> 
> Given that the
machines my students are using are all dual core, I started
>
working on a simple example that I hoped would show a nice speedup from
> the
> "easy" library.
> 
> The
specific problem I'm working on is a 2-d solution to the laplace
>
equation (electrostatics).  The bulk of the computation is a recursion
> relation, applied to elements of a 2-d array, according to the
following
> snippet.
> 
> Of course, by now I
should know that "simple" never really is.  When I
>
compile with gfortran and run with 1 or 2 cores (ie, OMP_NUM_THREADS=2,
> export OMP_NUM_THREADS) there is basically no difference in
execution
> time.
> 
> 
> Any suggestions?
 I figured that this would be a simple example to
> parallelize. 
Is there a better example for OpenMP parallelization?  Also,
> is
there something obvious I'm missing in the example below?
> 
> Nathan Moore
> 
> integer,parameter::Nx=1000
> integer,parameter::Ny=1000
> real*8 v(Nx,Ny)
>
integer boundary(Nx,Ny)
> 
> v_cloud = -1.0e-4
>
v_ground = 0.d0
> 
> convergence_v =
dabs(v_ground-v_cloud)/(1.d0*Ny*Ny)
> 
> ! initialize the
the boundary conditions
> do i=1,Nx
>         do j=1,Ny
>                 v_y = v_ground + (v_cloud-v_ground)*(j*dy/Ly)
>                 boundary(i,j)=0
>                 v(i,j) =
v_y
>                 ! we need to ensure that the edges of the
domain are held
> as
> boundary
>                
if(i.eq.0 .or. i.eq.Nx .or. j.eq.0 .or. j.eq.Ny) then
>           
             boundary(i,j)=1
>                 endif
>    
    end do
> end do
> 
> 10 converged = 1
> !$OMP PARALLEL
> !$OMP DO
>         do i=1,Nx
>                 do j=1,Ny
>                        
if(boundary(i,j).eq.0) then
>                                
old_v = v(i,j)
>                                 v(i,j) =
> 0.25*(v(i-1,j)+v(i+1,j)+v(i,j+1)+v(i,j-1))
>               
                 dv = dabs(old_v-v(i,j))
>                        
        if(dv.gt.convergence_v) then
>                            
            converged = 0
>                                
endif
>                         endif
>                
end do
>         end do
> !$OMP ENDDO
> !$OMP END
PARALLEL
> if(converged.eq.0) then
>         goto 10
> endif
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