[petsc-dev] SNES, Coloring, DM, and TS

[Dmitry Karpeev]

On Tue, Feb 28, 2012 at 2:06 PM, Jed Brown <jedbrown at mcs.anl.gov> wrote:

> On Tue, Feb 28, 2012 at 13:52, Dmitry Karpeev <karpeev at mcs.anl.gov> wrote:
>
>> I don't understand the need for multiple function evaluation routines and
>> their explicit naming.
>> I think the situation here is completely analogous to the situation with
>> matrices: I thought we agreed that each DM would be capable of generating a
>> single matrix, the "Jacobian", rather than multiple matrices like J and
>> J_pre, and that the different necessary matrices (again, e.g., J and J_pre)
>> would instead be produced by their respective DMs -- the KSP DM and the PC
>> DM, so to speak.  These subDMs would be pulled out of the SNES DM and
>> passed to the corresponding KSP and PC objects.  Thus, a DM encapsulates a
>> single "problem", and if we want to get a different problem -- the "exact"
>> linearization of the original, for example -- we extract the KSP DM;
>>  likewise, PC DM is an approximate linearization, and so on.
>>
>> I don't think TS is any different -- we can have the TSI DM (maybe under
>> a better name), which could have DMFormFunction encapsulating the IFunction
>> (and, incidentally, a linearization encapsulating the mass matrix), etc.
>>
>
> I'm concerned that this becomes insanity. A single TS might need
> RHSFunction, IFunction, and IJacobian. They might also partition terms
> differently for the functions and the Jacobian. Having different DMs seems
> awfully confusing.
>
> In short -- I'm for each DM having one function (DMFormFunction or,
>> better, DMFormResidual) and one matrix (DMFormJacobian) -- *a*
>> linearization of the residual.  All other functions and matrices are
>> obtained from the corresponding subDMs.
>>
>
> The interface is actually different for these (compare TSIFunction, a
> transient residual, to SNESFunction)
>

Just out of curiosity: how many transient problems have we seen where the
transient residual doesn't split like R(U,U_t) = R_0(U_t) + R_1(U)?  There
is the issue of explicit dependence on time, but if that could be
addressed, the problem would naturally split and, as a side benefit, there
would be a natural "mass" matrix coming out of the linearization of R_0.

Dmitry.
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