Project function while using PETScOptions.set('pc_type', 'fieldsplit')

Question

I'm using the PETSc fieldsplit preconditioner to solve my problem, defining fields via pestc4py with a command like:

is0 = PETSc.IS().createGeneral(V.sub(0).dofmap().dofs())
is1 = PETSc.IS().createGeneral(V.sub(1).dofmap().dofs())

where

V = MixedFunctionSpace([V_s, V_vec])

That works fine, but when I go to use the project function on a variable 'c' for example:

project(c, V_s)

I get an error. I think the reason is that project is actually a solve command, and it is still trying to apply the fieldsplit pc despite the other field not being included. If I bracket the project command with

PETScOptions.set('pc_type','asm')    
project(c,V_s)
PETScOptions.set('pc_type','fieldsplit')

it works but it is messy. Is there a simpler way?

Answer 1

One option may be to use prefixed option and set KrylovSolver's petsc_prefix parameter. But it is tedious to debug.

Cleaner way is to avoid using PETSc options at all and configure your Krylov solver using petsc4py.

Comments

Thanks Jan, I tried using petsc4py to configure my main snes solver but found it buggy / not fully capable of doing what I wanted, especially with fieldsplit.

I see what you mean re: the prefix parameter although it looks even less elegant that bracketing the 'project' function. I guess I was hoping for something akin to 'interpolate(c,V_s)' where it would just give me a Function with the local 'c' value at those coordinates.