[petsc-dev] Slowness of PetscSortIntWithArrayPair in MatAssembly

[Fande Kong]

BTW,

PetscSortIntWithArrayPair is used in MatStashSortCompress_Private.

Any way to avoid to use PetscSortIntWithArrayPair in
MatStashSortCompress_Private?

Fande,

On Tue, Jul 2, 2019 at 11:09 AM Fande Kong <fdkong.jd at gmail.com> wrote:

> Hi Developers,
>
> John just noticed that the matrix assembly was slow when having sufficient
> amount of off-diagonal entries. It was not a MPI issue since I was  able to
> reproduce the issue using two cores on my desktop, that is, "mpirun -n 2".
>
> I turned  on a profiling, and 99.99% of the time was spent
> on PetscSortIntWithArrayPair (recursively calling).   It took THREE MINUTES
>  to get the assembly done. And then changed to use the option
> "-matstash_legacy" to restore
> the code to the old assembly routine, and the same code took ONE SECOND to
> get the matrix assembly done.
>
> Should write any better sorting algorithms?
>
>
> Fande,
>
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