While reading my post again, I realized I did not have -snes_mf or -snes_mf_operator set with -ksp_only. This possibly explains the behavior I observed. I was not thinking this combination. I will confirm this tomorrow.
Thanks, Yaqi On Wed, Oct 14, 2015 at 1:08 AM, Yaqi Wang <[email protected]> wrote: > Hello All, > > This is possibly a newbie question but I cannot figure out why. A SNES is > set up in my code but I would like to use it for solving a simple linear > problem. > > The sequence of setting up the SNES is as the following: > SNESCreate() > SNESSetFunction() > SNESSetJacobian() > SNESSetFromOptions() > > I do not list all calls but just list those I think matters. Note that my > Jacobian is not exact, i.e. it is not consistent with my function. > > Then SNESSolve() is called. If I use -snes_type newtonls and > -snes_mf_operator, SNES can solve the linear problem properly, i.e. the > residual norm dropped bellow the tolerance I specified. I know I donot have > to by playing with tolerances, but I tried -snes_type ksponly. I saw my > residual function is only called at the beginning and the end of solve. > Most likely KSP is trying to solve the linear problem with the Jacobian I > specified, which is not exact. I cannot make the solution converge probably > because of this. > > My question is why KSPONLY is not using my function for evaluating (b-Ax) > at each linear iteration? I am using the default KSP type, which I guess > should be GMRES. I do not think it matters anyway. > Did I do something wrongly or I had some misunderstanding? > > Many thanks, > Yaqi > > >
