> On Aug 25, 2015, at 4:50 PM, Matthew Knepley <[email protected]> wrote: > > On Tue, Aug 25, 2015 at 4:44 PM, Barry Smith <[email protected]> wrote: > > > On Aug 25, 2015, at 3:22 PM, Matthew Knepley <[email protected]> wrote: > > > > On Tue, Aug 25, 2015 at 12:13 PM, Mark Adams <[email protected]> wrote: > > > > > > On Sat, Aug 22, 2015 at 10:39 PM, Barry Smith <[email protected]> wrote: > > > > > On Aug 22, 2015, at 9:26 PM, Mark Adams <[email protected]> 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 <[email protected]> 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
