perform only 1 residual calculation if solve_type=Linear

[WilkAndy]

Rationale

Only 1 residual calculation per timestep is needed if solve_type=Linear.

Description

At the moment two residual calculations are performed every timestep. One for the first nonlinear residual (this is required), then PETSc does its linear solve, and then another, redundant, nonlinear residual calculation. I do not know if it is PETSc, libmesh or MOOSE that is requesting this second residual calculation.

Impact

Computational efficiency. In the simple physics i happen to be modelling right now, just doing 1 residual calculation would speed up MOOSE by two times!

[fdkong]

OK. The second residual evaluation is added because we are asking PETSc to have a Nonlinear monitor by default. It is not necessary for linear problems. I think the simple solution is to use -snes_monitor_cancel . Then PETSc won't call the residual evaluation again. We may not care about the nonlinear residuals if the problems are linear.

It does not work for you, please kindly let me know.

[permcody]

Thanks @fdkong for that handy workaround. Is it worth adding an option to MOOSE for truly linear problems? Is there a wider framework change that might benefit people who know they are working with linear problems? I know that libMesh has different system types but you are correct we always assume nonlinear in MOOSE. I'm open to ideas here.

[fdkong]

solve_type = linear is already a pure linear solver except that we still have some nonlinear monitors. If we cancel out all nonlinear monitors then we end up a pure linear solver which should not have any extra work.