On Fri, Feb 28, 2014 at 4:48 PM, Barry Smith <[email protected]> wrote:
> > On Feb 28, 2014, at 3:27 PM, Xiangdong <[email protected]> wrote: > > > > > > > > > On Fri, Feb 28, 2014 at 4:21 PM, Matthew Knepley <[email protected]> > wrote: > > On Fri, Feb 28, 2014 at 1:16 PM, Xiangdong <[email protected]> wrote: > > If I assembly the matrix with MatSetValuesStencil and use > KspSetOpreators for Ksp, do I need to call KspSetDM first? > > > > No > > > > What is the key difference or advantage of using KspSetComputeOperators > against KspSetOperators? > > > > With the later, you have to manage creating and preallocating the matrix. > > > > If I use DMCreateMatrix and MatSetValuesStencil, I do not need to > preallocate the matrix. > > Correct. > > The SetComputeOperators and KSPSetDM is useful if you wish to use > multigrid on system since PCMG will call your compute operators function on > each level for you automatically. See src/ksp/ksp/examples/tutorials/ > ex25.c ex28.c ex29.c ex31.c ex32.c ex34.c ex45.c ex50.c If you are just > solving the one system and not using multigrid then there is no particular > advantage in SetComputeOperators and KSPSetDM > If I want to use multigrid as a preconditioner, I have to call KspSetComputeOperators. Is this true? Thank you. Xiangdong > > Barry > > > > > Xiangdong > > > > > > > > > > Matt > > > > Thanks you. > > > > Xiangdong > > > > > > On Tue, Feb 25, 2014 at 4:24 PM, Jed Brown <[email protected]> wrote: > > Xiangdong <[email protected]> writes: > > > I am not clear about this. In this example, where is the format > declaration > > > (e.g.,mpiaij) of Mat jac? Why is this mat jac always distributed in > > > a compatible way as the DM vectors? > > > > src/ksp/ksp/examples/tutorials/ex25.c calls KSPSetDM and sets functions > > to assemble the matrices and right hand side. Those objects are created > > internally (you can use -dm_mat_type sbaij if you like) and the user > > doesn't have to see them. > > > > > > > > > > -- > > 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 > > > >
