[petsc-users] SNES without SNESSetJacobian, snes_fd or snes_mf

[Florian Kauer]

Now everything makes sense, thank you!

I actually had the problem that if I have an unconnected node, it is not 
taken into account when calculating without -snes_fd. So that is 
probably due to the coloring based Jacobian calculation.

And the actual problem was that I did not completely follow Shri's 
advise from the "Extra Variable in DMCircuit" thread from 2014 to 
actually connect this node to the network.... With -snes_fd it was just 
working anyway so I did not care until I looked at the code again yesterday.

So, when connecting the node everything runs smoothly :-)


On 09.11.2017 16:38, Jed Brown wrote:
> Florian Kauer <florian.kauer at koalo.de> writes:
> 
>> Hi,
>> what is the SNES solver actually doing when you do not provide a
>> jacobian and not explicitly select either finite differencing
>> approximation or matrix-free Newton-Krylov method?
>>
>> I just noticed that I mistakenly did this, but a good solution is found
>> anyway (and fast). So what is actually happening? Simple fixed-point
>> iteration?
> 
> Presumably you are using a DM that can provide a coloring, in which case
> the sparse Jacobian is assembled using finite differencing with coloring.
>