On Wed, Mar 21, 2018 at 9:07 AM, 我 <[email protected]> wrote: > Thanks for your reply! You mean the preconditioner must be the necessary > choice for the linear iterative method in PETSc? >
No. As you saw, you can use no preconditioner. The bad convergence has nothing to do with PETSc. It is a mathematical fact. All iterative methods behave this way. > And the default preconditioner in PETSC is which one? > ILU(0). > I want to compare them in order to illustrate that PCHYPRE is best one for > my problem. > Then the right thing to do is read some papers and reproduce what other people have done, and show that Hypre is better than that. > If I want to get the matrix after preconditioned (e.g. PAx=Pb, and I want > to get PA), is there a function in PETSc? > http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/KSP/KSPComputeExplicitOperator.html It is extremely expensive and should only be used for very small problems. The whole idea of iterative methods is that you do NOT compute this operator explicitly. Matt > Thanks again! > Daye > > > > > At 2018-03-21 18:45:18, "Matthew Knepley" <[email protected]> wrote: > > On Wed, Mar 21, 2018 at 3:35 AM, 我 <[email protected]> wrote: > >> Hi, >> I want to compare the time cost between preconditioner and >> unpreconditioner in PETSc. But I didn't know how to turn off the >> preconditioner in Petsc. If I choose the PCNONE, but the solution even can >> not converge. >> > > That is how you turn off a preconditioner, -pc_type none. Without a > preconditioner, almost nothing converges. You can't have it both ways. > > Thanks, > > Matt > > >> If I do not declare PC at the beginning of my program, will PETSc choose >> a default preconditioner? I just want to turn off it. Any suggestions? >> Thank you very much! >> Daye >> >> >> >> > > > > -- > What most experimenters take for granted before they begin their > experiments is infinitely more interesting than any results to which their > experiments lead. > -- Norbert Wiener > > https://www.cse.buffalo.edu/~knepley/ <http://www.caam.rice.edu/~mk51/> > > > > > -- What most experimenters take for granted before they begin their experiments is infinitely more interesting than any results to which their experiments lead. -- Norbert Wiener https://www.cse.buffalo.edu/~knepley/ <http://www.caam.rice.edu/~mk51/>
