Performance issue with MatSetValues

[Christophe Picard]

Hi,

I have a performance issue while trying to insert values in the a matrix. I am 
using DMMG solver for cell-centered scheme in 3D from the petsc-snapshot to 
solve a Poisson equations. Inserting coefficients in the matrix for dirichlet 
or neumann boundary conditions, the insertion is instantaneous. But is I want 
to insert coefficients for periodic boundary conditions, I can notice a huge 
slow down in the insertion process (not in the resolution though).
The smallest size I can notice the performance drop is 32*32*32.

Is there any way to improve this? 


col.i  = row.i+(mx-1);
col.j  = row.j;
col.k = row.k;

v[0] =  1;

MatSetValuesStencil(*A,1,&row,1,&col,v,ADD_VALUES);



Thans,

Christophe