[petsc-users] KSP diverging

Matthew Knepley knepley at gmail.com
Fri Dec 23 19:18:12 CST 2011

On Fri, Dec 23, 2011 at 7:14 PM, Mohamad M. Nasr-Azadani
<mmnasr at gmail.com>wrote:

> Thank you guys,
> What happens right now when I use GMRES is that it reaches the maximum
> number of iterations I set, i.e. 1000 (which should not). I have set the
> restart to 20 (and then increased it to 1000) but no luck. It never
> converges.

It sounds like your operator is singular.


> On Fri, Dec 23, 2011 at 7:23 AM, Jed Brown <jedbrown at mcs.anl.gov> wrote:
>> On Fri, Dec 23, 2011 at 07:43, Matthew Knepley <knepley at gmail.com> wrote:
>>> Even if you lose orthogonality, it will just stagnate since you always
>>> have the solution
>>> from the first few vectors. Likewise, I do not see how you can get an
>>> diverging sequence
>>> from restarting since you always have the last residual, you just
>>> stagnate.
>> The true residual can grow even though the estimate of the preconditioned
>> residual decreases monotonically. When you restart, you re-evaluate the
>> actual residual, which might be much larger.

What most experimenters take for granted before they begin their
experiments is infinitely more interesting than any results to which their
experiments lead.
-- Norbert Wiener
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