[petsc-users] A quick question on 'un-symmetric graph'

[Zhenglun (Alan) Wei]

Dear folks,
      Here I came across a problem.

[0]PETSC ERROR: Petsc has generated inconsistent data!
[0]PETSC ERROR: Have un-symmetric graph (apparently). Use 
'-pc_gamg_sym_graph true' to symetrize the graph or '-pc_gamg_threshold 
0.0' if the matrix is structurally symmetric.!

     My code basically uses PETSc /src/ksp/ksp/example/tutorial/ex45.c 
to solve the Poisson equation with the Dirichlet BC in x-direction and 
the Periodic BC in y- and z- direction.
     The executable file is:

mpiexec -f $PBS_NODEFILE -np 32 ./ex45 -pc_type gamg -ksp_type cg 
-pc_gamg_agg_nsmooths 1 -mg_levels_ksp_max_it 1 -mg_levels_ksp_type 
richardson -ksp_rtol 1.0e-7


     There is no problem for my code when I use small computational 
domain (83*41*21 with single core and even 163*81*41 with 4 processes). 
However, when I increase the domain size (323*161*81 with 32 processes), 
the error comes up. I wonder what's the possible reason of this kind of 
problem. Do you need any of my output in order to do some further 
inspection? or I just need to blindly add '-pc_gamg_sym_graph true' in 
PETSc option?

thanks,
Alan