[Omp] Newbie in deep end >> Matrix multiplier > Need to add OpenMPdirectives

[Meadows, Lawrence F]

Non-trivial.

Here's matrix multiply in Fortran: B(n,l) * C(l,m) = A(n,m)
(actually, this adds the product of B and C to A, but
this is the important bit):

do i = 1,n
   do j = 1,m
      do k = 1,l 
         a(i,j) = a(i,j) + b(i,k) * c(k,j)
      enddo
    enddo
enddo

You can parallelize any loop that doesn't carry a dependence.
So that would be either outer loop (extra credit: what OpenMP
directive would you use to parallelize the innermost loop?)
The outermost loop is the best because of granularity, considering
only parallelism. So you would do:

!$omp parallel do
do i = 1,n
   do j = 1,m
      do k = 1,l 
         a(i,j) = a(i,j) + b(i,k) * c(k,j)
      enddo
    enddo
enddo
!$omp end parallel do

The last directive is not required.

In C, you would say:

#pragma omp parallel for

and translate the fortran to C, and probably put curlys around
the whole mess for clarity.

So the homework question is why is this a really bad way to
do matrix multiply, even ignoring the OpenMP directives? Once
you've figured that out, which I presume that you have, and
presumably discovered tiling, then you have to decide which
loop or loops in the tiled loop nest to parallelize. I suspect
that the answer is the outermost block loop, but I have now
exhausted the limits of my knowledge of matrix multiply for
this semester.

Regards,

Larry Meadows

>-----Original Message-----
>From: Omp-bounces at openmp.org [mailto:Omp-bounces at openmp.org] 
>On Behalf Of Bono Mitch
>Sent: Thursday, April 07, 2005 12:30 PM
>To: Omp at openmp.org
>Subject: [Omp] Newbie in deep end >> Matrix multiplier > Need 
>to add OpenMPdirectives
>
>Hi,
>
>I'm working on a matrix-matrix multiplier in C for my 
>dissertation. It is 
>based on the DGEMM fortran routine. The multiplier works great 
>but I need to 
>add open mp directives and I'm totally new to it so don't 
>really have a clue 
>where. I've looked at the FAQ's and the specifcation examples 
>but still 
>don't know where to add them.
>
>Can any of you point me to some examples where OpenMP has been 
>used in this 
>manner?
>
>Thanks for your help,
>
>Colly Mitch.
>
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