[petsc-users] Preallocation (dnz, onz arrays) in sparse parallel matrix

[Thibaut Appel]

Dear PETSc users,

I am trying to assemble a sparse matrix in parallel where my main 
objective is efficiency and scalability.

Precisely, I am using MatMPIAIJSetPreallocation with diagonal entries 
(dnz) and off-diagonal entries (onz) _arrays_ (non zero elements for 
each rows) to allocate the memory needed.

Prior to the insertion of the elements in the matrix, I am doing a 
primary loop to determine those arrays dnz and onz for each processor 
owning its own set of rows. Ideally,  this loop would look like

     for irow=istart,  iend-1,  i++ ----> count dnz(irow) and onz(irow)

But it seems that you cannot call 
MatGetOwnershipRange(Mat,istart,iend,ierr) before 
MatMPIAIJSetPreallocation to get istart and iend. Why is that?

Which optimal approach should be followed to count your non-zero 
elements for each processor? I saw two conversations where Barry Smith 
suggested the use of MatPreallocateInitialize/Finalize or 
PetscSplitOwnership, which means you have to determine yourself the rows 
owned by each processor? Is that not contrary to the "PETSc spirit"?

Thanks for your help and have a nice weekend

Thibaut

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