[petsc-dev] Slowness of PetscSortIntWithArrayPair in MatAssembly

[Fande Kong]

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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