[petsc-users] Problem about Residual evaluation

[Yingjie Wu]

Dear PETSc developers:
Hi,

I've been using -snes_mf_operator and I've customized a precondition matrix
to solve my problem.I have two questions about the residuals of linear
steps(KSP residual).


1.Since I'm using a matrix-free method, how do we get KSP residuals in
PETSc?

r_m = b - A*x_m

Is finite difference used to approximate "A*x_m" ?


2.What is the difference between instruction ' -ksp_monitor ' and '
-ksp_monitor_true_residual ' in how they are calculated?


Thanks,

Yingjie
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