[petsc-users] Parallelization of the code

[Matthew Knepley]

On Tue, Feb 12, 2019 at 6:43 PM Maahi Talukder via petsc-users <
petsc-users at mcs.anl.gov> wrote:

>
> Dear All,
>
>
> I am tying to solve a linear system using ksp solvers. I have managed to
> solve the system with a sequential code. The part of my sequential code
> that deals with creating Matrix and setting values is as the following -
>
> call MatCreate(PETSC_COMM_WORLD,Mp,ierr)
> call MatSetSizes(Mp,PETSC_DECIDE,PETSC_DECIDE,u*v,u*v,ierr)
> call MatSetFromOptions(Mp,ierr)
> call MatSetUp(Mp,ierr)
>
> Do p = 1,29008
> Do r = 1,29008
> if(Q(p,r)/=0.0) then
> val(1) = Q(p,r)
> col(1) = r-1
> call MatSetValues(Mp,ione,p-1,ione,col,val,INSERT_VALUES,ierr)
> endif
> end Do
> end Do
>
> call MatAssemblyBegin(Mp,MAT_FINAL_ASSEMBLY,ierr)
> call MatAssemblyEnd(Mp,MAT_FINAL_ASSEMBLY,ierr)
>
> And the part of my sequential code that creates the vector is -
>
> call VecCreateMPI(PETSC_COMM_WORLD,PETSC_DECIDE,u*v,Bx,ierr)
> call VecSetFromOptions(Bx,ierr)
> call VecDuplicate(Bx,Xp,ierr)
> call VecSet(Bx,zero,ierr)
>
> Do p = 1,29008
> if(Fx(p,1)/=0.0) then
> val(1) = Fx(p,1)
> call VecSetValues(Bx,ione,p-1,val,INSERT_VALUES,ierr)
> endif
> end Do
>
> call VecAssemblyBegin(Bx,ierr)
> call VecAssemblyEnd(Bx,ierr)
>
> So when I run the code on single processor, it runs fine. But when I tried
> to run it on more than one processor, it failed.
>

How? We cannot guess how it fails. You have to send the ENTIRE error
message.

Looking about it should not fail, but it will be really inefficient because
every process sets every value in the matrix and vector.

   Matt


> Now from what I understood from going through the manual is that if I use
> MatCreate to create my Matrix, then depending on the no of processor  that
> I put in after 'mpiexec -n ...' , it either acts either as a sequential
> code or a parallel code. And I don't need to anything extra to make it work
> in parallel, as PETSc does that internally.
>
> So would you please let me know  what modifications I need to do to my
> existing sequential code to make it work in parallel like using
> MatGetOwnershipRange ?
>
> Regards,
> Maahi Talukder
> MSc Student
> Clarkson University
>
>
>
>

-- 
What most experimenters take for granted before they begin their
experiments is infinitely more interesting than any results to which their
experiments lead.
-- Norbert Wiener

https://www.cse.buffalo.edu/~knepley/ <http://www.cse.buffalo.edu/~knepley/>
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