[petsc-users] How to confirm the performance of asynchronous computations

[Viet H.Q.H.]

Hello Petsc developers and supporters,

I would like to confirm the performance of asynchronous computations of
inner product computation overlapping with matrix-vector multiplication
computation by the below code.


 PetscLogDouble tt1,tt2;
    KSP ksp;
    //ierr = VecSet(c,one);
    ierr = VecSet(c,one);
    ierr = VecSet(u,one);
    ierr = VecSet(b,one);

    ierr = KSPCreate(PETSC_COMM_WORLD,&ksp); CHKERRQ(ierr);
    ierr = KSP_MatMult(ksp,A,x,Ax); CHKERRQ(ierr);


    ierr = PetscTime(&tt1);CHKERRQ(ierr);
    ierr = VecNormBegin(u,NORM_2,&norm1);
    ierr = PetscCommSplitReductionBegin(PetscObjectComm((PetscObject)Ax));
    ierr = KSP_MatMult(ksp,A,c,Ac);
    ierr = VecNormEnd(u,NORM_2,&norm1);
    ierr = PetscTime(&tt2);CHKERRQ(ierr);

    ierr = PetscPrintf(PETSC_COMM_WORLD, "The time used for the
asynchronous calculation: %f\n",tt2-tt1); CHKERRQ(ierr);
    ierr = PetscPrintf(PETSC_COMM_WORLD,"+ |u| =  %g\n",(double) norm1);
CHKERRQ(ierr);


    ierr = PetscTime(&tt1);CHKERRQ(ierr);
    ierr = VecNorm(b,NORM_2,&norm2); CHKERRQ(ierr);
    ierr = KSP_MatMult(ksp,A,c,Ac);
    ierr = PetscTime(&tt2);CHKERRQ(ierr);

    ierr = PetscPrintf(PETSC_COMM_WORLD, "The time used for the synchronous
calculation: %f\n",tt2-tt1); CHKERRQ(ierr);
    ierr = PetscPrintf(PETSC_COMM_WORLD,"+ |b| =  %g\n",(double) norm2);
CHKERRQ(ierr);


On a cluster with two or four nodes, the asynchronous computation is always
much slower than synchronous computation.

The time used for the asynchronous calculation: 0.000203
+ |u| =  100.
The time used for the synchronous calculation: 0.000006
+ |b| =  100.

Are there any necessary settings on MPI or Petsc to gain performance of
asynchronous computation?

Thank you very much for anything you can provide.
Sincerely,
Viet.
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