Thanks a lot, I expected something like that. I will look in this direction.
Best Timothée 2015-09-28 21:17 GMT+09:00 Matthew Knepley <[email protected]>: > On Mon, Sep 28, 2015 at 2:53 AM, Timothée Nicolas < > [email protected]> wrote: > >> Hi all, >> >> I have something strange in my application and I don't know what could >> cause this. I am trying to do an implicit MHD problem and I thought I >> finally figured out the preconditioner step, but somehow I don't get the >> expected result, not even close. >> >> For the preconditioning, I am using an approximate Schur complement, >> which requires two relatively easy KSP inversions at each preconditioner >> application. I apply this algorithm directly to the result function at the >> end of the routine FormFunction. I have checked that the approximation to >> the inversion of the Jacobian is good, in the sense that when I multiply >> the preconditioned vector by the *total* Jacobian matrix, I indeed >> recover almost the initial unpreconditioned vector. Also, I know that my >> Jacobian matrix is correct, because (i) I have checked manually that F(X + >> dX) ~ F(X) + J * dX and (ii) when I don't use -snes_mf and use the provided >> Jacobian matrix the result is pretty much equivalent to using -snes_mf. >> >> In my understanding, this means that what I effectively feed to SNES at >> the end of my FormFunction routine is a good approximation to J^(-1) F. As >> a result, I naturally expect that the number of KSP iterations necessary to >> achieve one SNES iteration be drastically reduced. However, I observe >> virtually no change whatsoever in the number of iterations. >> >> Any thoughts about what I could be missing ? Maybe I forgot to set a SNES >> or KSP option somewhere ? I can send pieces of code if needs be. >> > > It sounds like you are putting this in the wrong place. If you have the > action of a good preconditioner for the Jacobian, then you > should use a PCSHELL and pass it to the KSP. It does not belong in the > FormFunction. > > Thanks, > > Matt > > >> Best >> >> Timothee >> >> >> > > > -- > 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 >
