[petsc-dev] PetscObjectViewFromOptions() pull request 1005 1006

[Lawrence Mitchell]

> On 28 Jun 2018, at 17:24, Lisandro Dalcin <dalcinl at gmail.com> wrote:
> 
> On Thu, 28 Jun 2018 at 19:16, Lawrence Mitchell <wence at gmx.li> wrote:
>> 
>> This uses the Push/Pop stuff that I added in PR#604, in response to noting (in PCPATCH runs) that 90% of the time was spent in XXXViewFromOptions.  After adding the push/pop pair, this dropped to basically 0%.  I didn't do further exhaustive benchmarking to check for further problems.
>> 
> 
> There you have! Thanks for confirming my previous guess. I would
> really like that PR being split, and leave the changes to
> XXXSetFromOptions() on hold in a separate PR.

OK, I've done some more benchmarking now, and cooked up a very simple test case.  I just solve a tiny problem 10 million times.

On master, this completes on my machine in:

If I leave the viewers on, this takes ages:

(master-viewers-on) $ time taskset -c 1 ./many-ksps
real 0m37.07s

When I turn the viewers off
(master-viewers-off) $ time taskset -c 1 ./many-ksps
real 0m17.979s

On knepley/feature-pc-patch both are faster, with no difference between turning viewers on and turning viewers off.

(patch) $ time task set -c 1 ./many-ksps
real 0m12.512s

So a 25%-30% win.

IOW, Patrick and I would definitely like this change to make it into master (it has a noticeable effect on the speed of our solver).  But it could arguably be split in two.

Lawrence

#include <petsc.h>

int main(int argc, char **argv)
{
     PetscErrorCode ierr;
     Vec x, y;
     KSP ksp;
     Mat A;
     MPI_Comm comm;
     ierr = PetscInitialize(&argc, &argv, NULL, NULL);
     if (ierr) return ierr;

     comm = PETSC_COMM_WORLD;

     ierr = MatCreate(comm, &A); CHKERRQ(ierr);

     ierr = MatSetSizes(A, PETSC_DECIDE, PETSC_DECIDE, 20, 20); CHKERRQ(ierr);

     ierr = MatSetUp(A); CHKERRQ(ierr);

     for (int i = 0; i < 20; i++) {
          PetscScalar one = 1;
          ierr = MatSetValues(A, 1, &i, 1, &i, &one, INSERT_VALUES); CHKERRQ(ierr);
     }
     ierr = MatAssemblyBegin(A, MAT_FINAL_ASSEMBLY); CHKERRQ(ierr);
     ierr = MatAssemblyEnd(A, MAT_FINAL_ASSEMBLY); CHKERRQ(ierr);

     ierr = MatCreateVecs(A, &x, &y); CHKERRQ(ierr);

     ierr = KSPCreate(comm, &ksp); CHKERRQ(ierr);

     ierr = KSPSetOperators(ksp, A, A); CHKERRQ(ierr);

     ierr = KSPSetFromOptions(ksp); CHKERRQ(ierr);

     ierr = PetscOptionsPushGetViewerOff(PETSC_TRUE); CHKERRQ(ierr);
     for ( int i = 0; i < 10000000; i++ ) {
          ierr = VecSet(x, 0); CHKERRQ(ierr);
          ierr = VecSet(y, 1); CHKERRQ(ierr);
          ierr = KSPSolve(ksp, y, x); CHKERRQ(ierr);
     }
     ierr = PetscOptionsPopGetViewerOff(); CHKERRQ(ierr);

     ierr = KSPDestroy(&ksp); CHKERRQ(ierr);
     ierr = MatDestroy(&A); CHKERRQ(ierr);
     ierr = VecDestroy(&x); CHKERRQ(ierr);
     ierr = VecDestroy(&y); CHKERRQ(ierr);

     ierr = PetscFinalize();
     return ierr;
}