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

[Ben Tay]

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.