On Tue, Aug 25, 2015 at 5:09 PM, Barry Smith <[email protected]> wrote:
> > > 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. drand48() is POSIX, and its a 1 line algorithm. If you can dig up a system on which it is missing, I will implement a stopgap. Matt > > 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 > > -- 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
