[petsc-users] snes failures

[Barry Smith]

> On May 20, 2016, at 10:11 AM, Juha Jaykka <juhaj at iki.fi> wrote:
> 
>> not. This usually indicates a bad guess for Newton. I would now advocate
> 
> Oh, did I forget to mention, that other initial conditions DO converge. The 
> funny thing is the initial condition which does NOT converge is actually 
> CLOSER to the solution than ones which DO. Had it been the other way around, I 
> probably would not have thought twice of it.

   Yeah, "closeness" for convergence of Newton's method can be rather complicated.

   Send the code. Python though it may be.

  Barry

> 
>>  1) Grid sequencing: This is easy if you use a DMDA. You just use
>> -snes_grid_sequence
>>      and its automatic. Since you report that smaller grids converge, this
>> is usually enough.
> 
> Unfortunately, no.
> 
>>  2) NPC: If the above fails, I would try preconditioning Newton with GS.
>> You can do this
>>      just using multiplicative composition, or left preconditioning.
> 
> I'm not sure what you mean here. Do you mean I should run normal newton line 
> search snes with SNESSetNPC(snes_of_type_gs)?
> 
> If so, does the inner snes_of_type_gs and the outer snes_of_type_newtonls get 
> all the same routines? I guess so, but just checking.
> 
>> If 1 works, then its likely that FAS would work even better.
> 
> I wish it was so easy: I did try all the snes types before posting the first 
> post. Also all KSP types. Except those that need things I don't have, of 
> course.
> 
> Cheers,
> Juha