If I use MatISSetLocalMat with a preallocated MatCreateSeqAIJ local matrix, do I need to use MatISSetPreallocation for the global matrix ? Here is the pseudo-code: MatCreateIS(PETSC_COMM_WORLD, ..., &globalMat) MatISSetPreallocation(globalMatrix, ...) // Is this necessary ? MatCreateSeqAIJ(PETSC_COMM_SELF, ..., &localMatrix) // Prealloc done on the fly MatSetValues(localMatrix, ...) MatISSetLocalMat(globalMatrix, localMatrix) Is it necessary to call MatISSetPreallocation for globalMatrix ? (prealloc should have been done locally for each local matrix, no ?) Franck