To extend over what Wolfgang said, the most likely causes for failure
with the Chebyshev preconditioner are:
- the vector supplying the matrix diagonal (via the field
AdditionalData::preconditioner) contains Inf or NaN entries, which in
turn result from either a ill-formed differential operator or a bug in
the code computing the diagonal, or
- a coarse level holding only constrained degrees of freedom, which are
not treated properly so that the SolverCG making an eigenvalue estimate
needs to work with an unsolvable linear system (right hand side not in
the image of the matrix).
For the first case, you would check the diagonal on each level and the
code leading to that, in the second the treatment of constraints.
Best,
Martin
On 14.02.23 06:33, 'yy.wayne' via deal.II User Group wrote:
Thank you Wolfgang. I didin't dive much into the Chebyshev
preconditioner because I'm not familiar with it.
I assume it's because the CG solver inside Chebyshev preconditioner
didn't converge or what.
But as you stated, if NaN should only get from NaN elements and
divided by zero, then I may find the error.
I'll follow your debug suggestion.
Best
Wayne
在2023年2月14日星期二 UTC+8 12:48:42<Wolfgang Bangerth> 写道：
> It's strange that the iteration breaks at step 0. If I try right
> preconditioning, it breaks at step 1 with nan. Besides, if there
is only on
> multigrid level, the problem is solved correctly with 1
iteration, so matrices
> should be correct. The coarse solution is good enough for
smoothing.
>
> The question is, in which case may a CG or GMRES solver breaks
at step 0 with nan?
NaN errors, like segmentation faults, are relatively easy to find
because you
know that every NaN you get is wrong. So you check that the matrix
and right
hand side and solution vector you give to the solver don't have
any NaNs. If
they don't, and you get NaNs out, then the problem likely lies
with the
preconditioner, so you single step through the solver around the
place where
the preconditioner is applied and check in which operation the NaN
appears.
In your case, you seem to have already found out that it's the
Chebyshev
preconditioner. The question is why. Is it because the matrix the
preconditioner uses has NaNs in it? Is it because the
preconditioner makes an
assumption that it can divide by certain matrix entries, but they
are zero?
Single stepping in a debugger will give you the answer :-)
Best
W.
--
------------------------------------------------------------------------
Wolfgang Bangerth email: bang...@colostate.edu
www: http://www.math.colostate.edu/~bangerth/
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