Comparing between a PETSc matrix and a standard fortran array in compressed row format

[Barry Smith]

  You can use MatCreateSeqAIJWithArrays() to "convert" the NSPCG matrix into
PETSc format and then MatAXPY() to difference them and then MatNorm() to 
see how large the result is. 

  Make sure the PETSc or hypre solver is always converging. Run with 
-ksp_converged_reason and or -ksp_monitor. My guess is that the matrix is
becoming very ill-conditioned so the solvers with the default options are 
failing.

    Barry


On Fri, 3 Aug 2007, Ben Tay wrote:

> Hi,
> 
> I used 2 packages to solve my poisson eqn, which is part of my NS unsteady
> solver. One is the NSPCG solver package which uses the compressed row format
> to store the A matrix. The other is PETSc. I found that using both solvers
> gave me similar answers for a number of time step. However, after that, the
> answer will suddenly change drastically for the PETSc case. This does not
> happen for the NSPCG solver.
> 
> For e.g. time step 1-315, oscillating airfoil case, pressure changes smoothly,
> similar answers in both cases
> 
> at time=316, pressure changes from -3.22 to -3.2 for NSPCG, but pressure
> changes from -3.21 to -60.2 for PETSc
> 
> This happens when I use HYPRE's AMG or PETSc's direct solver LU.
> 
> I have been trying to find out what's the cause and I can't find the answer in
> debugging. I would like to compare the values of the matrix of the 2 different
> solvers and see if there's any difference. However, NSPCG's matrix is in
> compressed row format while PETSc's one is just an address and it can't be
> viewed easily. Moreover, it's a big matrix so it's not possible to check by
> inspection. I'm thinking of subtracting one matrix by the other and find if
> it's zero. What's the best way to solve this problem? Btw, I'm using fortran
> and there's no mpi
> 
> Thank you.
> 
> 
>