[petsc-users] Building the same petsc matrix with different numprocs gives different results!

[Jed Brown]

Analabha Roy <hariseldon99 at gmail.com> writes:

> Hi all,
>
>
> Compiling and running this
> code<https://code.google.com/p/daneelrepo/source/browse/eth_question/eth.c>that
> builds a petsc matrix gives different results when run with different
> number of processors.

Uh, if you call rand() on different processors, why would you expect it
to give the same results?

for (sitestride1 = Istart; sitestride1 < Iend; sitestride1++)
    {
      for (sitestride2 = 0; sitestride2 < matsize; sitestride2++)
        {
          for (alpha = 0; alpha < dim; alpha++)
            {
              for (mu = 0; mu < dim; mu++)
                for (lambda = 0; lambda < dim; lambda++)
                  {
                    vecval = rand () / rand ();
                  }
 
              VecSetValue (BDB_AA, alpha, vecval, INSERT_VALUES);
 
            }
          VecAssemblyBegin (BDB_AA);
          VecAssemblyEnd (BDB_AA);
          VecSum (BDB_AA, &element);
          colvalues[sitestride2] = element;
 
        }
      //Insert the array of colvalues to the sitestride1^th row of H   
      MatSetValues (AVG_BDIBJ, 1, &sitestride1, matsize, idx, colvalues,
                    INSERT_VALUES);
 
    }

> The code is large and complex, so I have created a smaller program
> with the same
> loop structure here. <http://pastebin.ca/2457643>
>
> Compile it and run it with "mpirun -np $N ./test -draw_pause -1" gives
> different results for different values of N even though it's not supposed
> to.

What do you expect to see?

> Here is a sample output <http://i42.tinypic.com/2s16ccw.jpg> for N=1,2,3,4
> from left to right.
>
> Can anyone guide me as to what I'm doing wrong? Are any of the petssc
> routines used not parallelizable?
>
> Thanks in advance,
>
> Regards.
>
> -- 
> ---
> *Analabha Roy*
> C.S.I.R <http://www.csir.res.in>  Senior Research
> Associate<http://csirhrdg.res.in/poolsra.htm>
> Saha Institute of Nuclear Physics <http://www.saha.ac.in>
> Section 1, Block AF
> Bidhannagar, Calcutta 700064
> India
> *Emails*: daneel at physics.utexas.edu, hariseldon99 at gmail.com
> *Webpage*: http://www.ph.utexas.edu/~daneel/
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