> On Aug 25, 2015, at 4:50 PM, Matthew Knepley <knep...@gmail.com> wrote:
>
> On Tue, Aug 25, 2015 at 4:44 PM, Barry Smith <bsm...@mcs.anl.gov> wrote:
>
> > On Aug 25, 2015, at 3:22 PM, Matthew Knepley <knep...@gmail.com> wrote:
> >
> > On Tue, Aug 25, 2015 at 12:13 PM, Mark Adams <mfad...@lbl.gov> wrote:
> >
> >
> > On Sat, Aug 22, 2015 at 10:39 PM, Barry Smith <bsm...@mcs.anl.gov> wrote:
> >
> > > On Aug 22, 2015, at 9:26 PM, Mark Adams <mfad...@lbl.gov> wrote:
> > >
> > > Good point. I can not see any reason to use the initial guess for the
> > > eigen estimate.
> >
> > Why not, won't it better select for the eigen space actually seen by the
> > linear solver since the linear solver starts with that guess? I am just
> > making this up because I haven't looked how the initial guess affects
> > eigenanalysis for Chebyshev it but ...
> >
> > I've not looked at it, and it would be hard to because it is not a well
> > defined problem. But, if the initial guess is low frequency then it will
> > give a poor estimate for the highest eigen value. It is not clear to me
> > what the relationship is, generally, of an initial guess and the solution,
> > spectrally. Initial guesses will change as the problem evolves but we
> > don't update the eigen estimates. If the user's initial guess happens to
> > be zero then god knows what happens. (This is actually the case for the
> > XGC1 code!!!) It adds one more variable in debugging AMG, which is hard
> > enough as it is.
> >
> >
> > > I would vote for (1).
> > >
> > > Also, I hope cheb->random is the default.
> >
> > Well then different machines will produce different convergence
> > histories which is annoying for any kind of "no change" daily testing.
> > Except for you, most of the rest of us don't like the random default, sorry
> > :-)
> >
> > We can use the determinate random number that I added to GAMG (eg, v(i) =
> > ((double)(gid(i)*51)%100) - 50.)/50.
> >
> > Wait, why are you doing this? Why not just create a PetscRandom and set the
> > seed?
>
> Yes, he should create a new PetscRandom implementation called for example
> deterministic that would do some simple minded thing. All the current
> PetscRandom implementations might create different numbers with different OS,
> or versions of OS thus making "no change" tests impossible.
>
> This makes no sense. drand48() is a 48-bit linear congruential random number
> generator. It should never return a different value given the same seed.
From the makefile
#requiresfunction 'PETSC_HAVE_DRAND48'
It appears that not all systems have drand48(). So it is still an issue of
making sure we have a way of generating the exact same numbers on all systems.
Barry
>
> Matt
>
> Barry
>
> >
> > Matt
> >
> >
> > > One of my apps uses a zero RHS for the first solve, just because they
> > > did not care about adding some logic like: if (.not. first_solve) solve()
> > > Using a zero RHS would be catastrophic.
> >
> > There code be a check that rhs norm is zero (cost of a global
> > reduction?) and then use a nonzero initial guess to do the eigenanalysis
> >
> > Too complicated for little gain, or loss even, and requires a reduction.
> >
> >
> > > I trust this is true, because the code works, but we should make sure.
> > > And perhaps Cheby should check that that KSPSolve did all of its
> > > iterations (ie, DIVERGE_ITS, or whatever). Getting this wrong leads to
> > > silent errors that are a pain to debug.
> >
> > Good point, the KSPChebyshevComputeExtremeEigenvalues_Private() routine
> > should check that n returned from KSPGetIterationNumber() is not zero etc.
> >
> > Barry
> >
> > So I think we should:
> >
> > 1) set the initial guess with my new sort of random number (we don't need
> > good quality random number here)
> > 2) add the check for DIVERGE_ITS, or something, in
> > KSPChebyshevComputeExtremeEigenvalues_Private. Should it stop? Probably.
> >
> > Mark
> >
> >
> > >
> > > I can do this.
> > >
> > > Mark
> > >
> > >
> > >
> > > On Sat, Aug 22, 2015 at 6:35 PM, Barry Smith <bsm...@mcs.anl.gov> wrote:
> > >
> > > From KSPSolve_Chebyshev()
> > >
> > > X = ksp->work[0];
> > > if (cheb->random) {
> > > B = ksp->work[1];
> > > ierr = VecSetRandom(B,cheb->random);CHKERRQ(ierr);
> > > } else {
> > > B = ksp->vec_rhs;
> > > }
> > > ierr = KSPSolve(cheb->kspest,B,X);CHKERRQ(ierr);
> > >
> > > if (ksp->guess_zero) {
> > > ierr = VecZeroEntries(X);CHKERRQ(ierr);
> > > }
> > > ierr =
> > > KSPChebyshevComputeExtremeEigenvalues_Private(cheb->kspest,&min,&max);CHKERRQ(ierr);
> > >
> > > This seems to do strange stuff with the initial guess for the
> > > eigenanalysis. ksp->work[0] is a work vector used within the Chebyshev
> > > algorithm, so at this point in the code it will have just whatever stuff
> > > it had in it from a previous Chebyshev solver or a zero the first time
> > > through. It seems bad to use this vector as the initial guess for
> > > estimator. Then AFTER the KSPSolve() it zeros ksp->work[0], sometimes?
> > > If the original system being solved has zero initial guess, even though
> > > the values in X will not be used again. WTF?
> > >
> > > Shouldn't the code either
> > >
> > > 1) zero X = ksp->work[0] everytime BEFORE the KSPSolve() or
> > > 2) zero X if ksp->guess_zero and otherwise copy into X the initial guess
> > > vec_sol from the caller before computing the eigenvalues to use that
> > > initial guess in estimating the eigenvalues?
> > >
> > > Barry
> > >
> > >
> >
> >
> >
> >
> >
> > --
> > 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
>
>
>
>
> --
> 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
