[petsc-users] Problem about Residual evaluation

[Zou, Ling]

For question 2, there are some discussions in PETSc’s manual section 4.3 and 4.3.3.
https://www.mcs.anl.gov/petsc/petsc-current/docs/manual.pdf

-Ling

From: petsc-users <petsc-users-bounces at mcs.anl.gov> on behalf of Yingjie Wu via petsc-users <petsc-users at mcs.anl.gov>
Reply-To: Yingjie Wu <yjwu16 at gmail.com>
Date: Monday, April 1, 2019 at 9:22 AM
To: "petsc-users at mcs.anl.gov" <petsc-users at mcs.anl.gov>
Subject: [petsc-users] Problem about Residual evaluation

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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