Yes - the default preconditioner is block-jacobi - with one block on each processor.
So when run on 1 proc vs 8 proc - the preconditioner is different (with 1block for bjacobi vs 8blocks for bjacobi)- hence difference in convergence. Satish On Wed, 17 Apr 2019, Marian Greg via petsc-users wrote: > Hi All, > > I am facing strange behavior of the ksp solvers with increasing number of > MPI. The solver is taking more and more iterations with increase in number > of MPIs. Is that a normal situation? I was expecting to get the same number > of iteration with whatever number of MPIs I use. > > E.g. > My matrix has about 2 million dofs > Solving with np 1 takes about 3500 iteration while solving with np 4 takes > 6500 iterations for the same convergence criteria. > > Thanks > Mari >