[petsc-dev] Slowness of PetscSortIntWithArrayPair in MatAssembly

[Zhang, Junchao]

Try this to see if it helps:

diff --git a/src/sys/utils/sorti.c b/src/sys/utils/sorti.c
index 1b07205a..90779891 100644
--- a/src/sys/utils/sorti.c
+++ b/src/sys/utils/sorti.c
@@ -294,7 +294,8 @@ static PetscErrorCode PetscSortIntWithArrayPair_Private(PetscInt *L,PetscInt *J,
     }
     PetscFunctionReturn(0);
   }
-  SWAP3(L[0],L[right/2],J[0],J[right/2],K[0],K[right/2],tmp);
+  i = MEDIAN(L,right);
+  SWAP3(L[0],L[i],J[0],J[i],K[0],K[i],tmp);
   vl   = L[0];
   last = 0;
   for (i=1; i<=right; i++) {


On Tue, Jul 2, 2019 at 12:14 PM Fande Kong via petsc-dev <petsc-dev at mcs.anl.gov<mailto:petsc-dev at mcs.anl.gov>> wrote:
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<mailto: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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