[petsc-dev] How to get subsnes for subproblems based field decomposition

[Barry Smith]

> On Aug 19, 2016, at 8:49 AM, Lulu Liu <lulu.liu at kaust.edu.sa> wrote:
> 
> Hi,
> 
> For example, I would like to solve the nonlinear driven cavity problem (ex19.c in SNES). There are four components: u,v, omega, T. 
> 
> I want to solve one equation at one time, is there any easier way to get subsnes (for subproblems) from original SNES.

   Here's the thing. If the example defines a single monolithic function that computes the unknowns entire function, in this example the function is called FormFunctionLocal() there there is no way for PETSc which is a library to "pull out of" that FormFunctionLocal() an function that computes say just the functions for u or v or omega or T. That would require compiler tools that understood the mathematics of the source code function written in C which is essentially impossible.

    The only hope to be able to build nonlinear solvers that work on "a component at a time" is if the user provides the unknowns mathematical function not as a single monolithic function but as a collection of functions one for each component. This is why there is no way to automatically generate the SubSNES's you desire. Note that for the linear case where a matrix is provided it is easy to pull out the sub-linear problems because they are just sub matrices of the original matrix.

   Barry

> 
> Thanks!
> 
> 
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