[Omp] Newbie in deep end >> Matrix multiplier > Need toaddOpenMPdirectives

[Bono Mitch]

Not on sun sorry - working on my own pc in linux env using intel compiler.

I took your advice Grant and I understand why that is but it's still slower.

Can you tell me from the code I gave you, if that is enough OpenMP - i mean 
is there much more I could do to it?

thanks,

Colly.


>
>Mitch,
>
>Grant is right. By default, 'i' and 'l' are shared among the threads and
>you will run into a data race condition.
>
>If you are compiling on Sun Solaris platform, you can use -xvpara option
>to watch for such errors.
>
>For example,
>
> >cat -n t.c
>      1	int main()
>      2	{
>      3	    int i,j;
>      4	    int a[100][100];
>      5
>      6	#pragma omp parallel for
>      7	    for (i=1; i<100; i++)
>      8	        for (j=1; j<100; j++)
>      9	            a[i][j] = 0;
>     10	}
> >cc -xopenmp -xO3 -xvpara -c t2.c
>"t.c", line 6: Warning: inappropriate scoping
>	variable 'j' may be scoped inappropriately as 'shared'
>	. read at line 8 and write at line 8 may cause data race
>
>
>Regards,
>
>Yuan
>
>
>
>
>
>Haab, Grant wrote:
> > Mitch,
> >
> > You must declare i and l private for this to work:
> >
> >
> > #pragma omp parallel for private(i,l)
> > for (j = 0; j < *n_cols; ++j) //col
> > {
> > ...
> >
> >
> > That should fix the problem of slower execution.  As written 
below,
> > all the threads are using the same iteration variables for the 
inner
> > loops, causing incorrect results and bad performance.  The j index 
is
> > made private
> > for you since it is the parallel loop index.  The rest you must 
make
> > private explicitly, OR declare inside the j-loop:
> >
> >
> > #pragma omp parallel for
> > for (j = 0; j < *n_cols; ++j) //col
> > {
> >     int i, l; 	// this makes i & l private
> >
> > ...
> >
> >
> >
> > - Grant
> >
> > -----Original Message-----
> > From: Omp-bounces at openmp.org [mailto:Omp-bounces at openmp.org] On 
Behalf
> > Of Bono Mitch
> > Sent: Thursday, April 07, 2005 4:52 PM
> > To: Meadows, Lawrence F; Omp at openmp.org
> > Subject: RE: [Omp] Newbie in deep end >> Matrix multiplier 
> Need to
> > addOpenMPdirectives
> >
> > Ok...
> > Here's what I've done, but I don't think it's right, simply 
because it's
> >
> > slower when set to 2 threads than it is with one.
> > My project is based on the standard O(n^3) algorithm so I'm not 
allowed
> > to
> > use the auto blocking or any of the other multiplication methods. 
Sorry
> > I
> > should have told you this already. Essentially I'm using the DGEMM
> > Fortran
> > routine > http://www.netlib.org/blas/dgemm.f
> >
> >
> >
> >
> >
> > #pragma omp parallel for
> > for (j = 0; j < *n_cols; ++j) //col
> > {
> > 	if (*beta == 0.)
> > 	{
> > 		for(i = 0; i < *m_rows; ++i) //row
> > 		{
> > 			matrixC[i + j * *ldc] = 0.;
> > 		}
> > 	}
> > 	else if (*beta != 1.)
> > 	{
> > 		for(i =0; i < *m_rows; ++i)//row
> > 		{
> > 			matrixC[i + j * *ldc] = *beta * matrixC[i + j *
> > *ldc];
> > 		}
> > 	}
> > 	for (l = 0; l < *k_common; ++l)
> > 	{
> > 		if (matrixB[l + j * *ldb] != 0.)
> > 		{
> > 			temp = *alpha * matrixB[l + j * *ldb];
> > 			for(i = 0; i < *m_rows; ++i)
> > 			{
> > 				matrixC[i + j * *ldc] += temp *
> > matrixA[i + l * *lda];
> > 			}
> > 		}
> > 	}
> > }
> > ...
> > more similar loops depending on whether the matrices A or B are to 
be
> > transposed.
> >
> > What do you think?
> >
> > Thanks,
> >
> > Colm
> >
> > >
> > >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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