[petsc-users] large number of iteration.

[Mark Adams]

* Is your matrix symmetric?

* I see you said: "> Then I use CG for KSP and algebraic multigrid for
precondiitoner. For a 150K dofs problem, the linear solver converges with
200s iteration(still too much I think)"

Is this with fracture? A simple cube elasticity test would be the best
place to start. Can we get back to this?



> Does '-ksp_type cg -pc_type jacobi' work?
>>
>
>       Yes. and the ksp monitor shows the same output if I ran the same
> problem with petsc's example.
>
>>
>> Does '-ksp_type chebyshev -ksp_chebyshev_esteig_noisy and -pc_type jacobi'
>> work?
>>
>
>      No. I get a NAN residual after 1000s iteration.
>

Well this is strange. I think that "noisy" must give you a lower eigen
estimate. If your eigen estimate is too low Cheby is not stable. Does this
1000 iterations diverge or does the Nan come out of nowhere?

You can check the eigen estimate with '-ksp_chebyshev_esteig_ksp_view' (I
think). This will print out solver info including these eigen estimate. Set
'-ksp_chebyshev_esteig_ksp_max_it 10' to get the solve to complete and get
the ksp_view printout. You can read this. I have to think that this
estimate is lower with "noisy".
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