Hello, in order to preallocate I have to know whether a non-zero will be in the local diagonal submatrix or not.
http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/Mat/MatMPIAIJSetPreallocation.html says: The DIAGONAL portion of the local submatrix of a processor can be defined as the submatrix which is obtained by extraction the part corresponding to the rows r1-r2 and columns c1-c2 of the global matrix, where r1 is the first row that belongs to the processor, r2 is the last row belonging to the this processor, and c1-c2 is range of indices of the local part of a vector suitable for applying the matrix to. This is an mxn matrix. In the common case of a square matrix, the row and column ranges are the same and the DIAGONAL part is also square. The remaining portion of the local submatrix (mxN) constitute the OFF-DIAGONAL portion. Therefore I compute the begin and end rows and cols like that: int ownerRangeBeginA = _matrixA.ownerRange().first; int ownerRangeEndA = _matrixA.ownerRange().second; // Uses: MatGetVecs(_matrixA.matrix, &vector, nullptr); petsc::Vector diagTest{_matrixA, "test", petsc::Vector::RIGHT}; int localDiagColBegin = diagTest.ownerRange().first; int localDiagColEnd = diagTest.ownerRange().second; Debug("Local Submatrix Rows = " << ownerRangeBeginA << " / " << ownerRangeEndA << ", Local Submatrix Cols = " << localDiagColBegin << " / " << localDiagColEnd); It's a little bit tainted by my PETSc C++ helper function, but I think you get the code. Is there a way to do it more elegantly? Without instantiating a vector just for the purpose of gettings its owner range? And then testing a if an index is inside the local diagonal submatrix? Thanks, Florian
