LinearSolver: update after ConvergenceError

[pbrubeck]

Description

Fixes #4143

[jrmaddison]

u.dat.data_ro is zero here, i.e. the zero initial guess, rather than the result from one CG iteration

[pbrubeck]

The initial guess is -1.

[jrmaddison]

I think it's zero, as ksp_initial_guess_nonzero defaults to False.

[pbrubeck]

LinearSolver used to just wrap a KSP, but it now became a NonlinearVariationalSolver, so we need to decide whether we want to preserve the old behavior or to accomodate to the usual way of providing initial guesses to NLVS and always use whatever is stored in the solution.

ksp_initial_guess_nonzero applies only to the Newton update step, that stores the update as an internal PETSc.Vec, which is reused across different solve() calls, so the default ksp_initial_guess_nonzero=False makes sense here.

[jrmaddison]

I didn't realize that was the behaviour. Does that mean a LinearVariationalSolver always uses a non-zero initial guess for the (single step) Newton solve, and then zero guesses the increment solve? i.e. ksp_initial_guess_nonzero has no effect for a LinearVariationalSolver?

[pbrubeck]

I didn't realize that was the behaviour. Does that mean a LinearVariationalSolver always uses a non-zero initial guess for the (single step) Newton solve, and then zero guesses the increment solve?

This is correct.

i.e. ksp_initial_guess_nonzero has no effect for a LinearVariationalSolver?

The effect is very subtle, and is only for the update solve, ksp_initial_guess_nonzero=True will use the previous update vector as the initial guess for the current update vector.

[pbrubeck]

Upon discussing this during the meeting, LinearSolver will use as initial guess whatever is passed to solve(x, b) to be consistent with NLVS and LVS. So this PR will only deal with alwats updating the solution, even when an Exception is raised.

[jrmaddison]

Upon discussing this during the meeting, LinearSolver will use as initial guess whatever is passed to solve(x, b) to be consistent with NLVS and LVS. So this PR will only deal with alwats updating the solution, even when an Exception is raised.

Sounds good, thanks.