On Fri, Mar 6, 2015 at 6:03 PM Barry Smith <[email protected]> wrote:
> > > On Mar 6, 2015, at 4:41 PM, Jed Brown <[email protected]> wrote: > > > > Dmitry Karpeyev <[email protected]> writes: > >> This is trickier than it might appear: nonzerostate effectively counts > the > >> global number of nonzeros. > > No it does not. Note in MatZeroRows_SeqAIJ() when entries are deleted > from the matrix we still increase the nonzerostate. > >From MatAssemblyEnd_MPIAIJ(): if ((!mat->was_assembled && mode == MAT_FINAL_ASSEMBLY) || !((Mat_SeqAIJ*)(aij->A->data))->nonew) { PetscObjectState state = aij->A->nonzerostate + aij->B->nonzerostate; ierr = MPI_Allreduce(&state,&mat->nonzerostate,1,MPIU_INT64,MPI_SUM,PetscObjectComm((PetscObject)mat));CHKERRQ(ierr); } Barring MatZeroRows MATSEQAIJ matrices aij->A and aij->B will simply increment their nonzerostates on each new nonzero insertion in MatSetVAlues_SeqAIJ(), so the containing MATMPIAIJ M ends up with the total number of nonzeros in its nonzerostate. Now I reset my M. That will blow away aij->A and aij->B. I can now insert the same number of nonzeros, but in a different pattern. M will end up with the same nonzerostate as before the reset and confuse the PC, no? This started out as a discussion about MatReset(), but I think this _may_ be a bug we are seeing in one of the elastic contact applications: PCASM tries to rebuild itself with MAT_REUSE_MATRIX when subdomain matrices actually have different numbers of nonzeros. I have to say I haven't ascertained that an inconsistent nonzerostate cases the problem, yet -- reproducible test cases that trigger the problem are still too big to debug. > > >> The PC will rebuild if its state is stale, but > >> it will reuse matrices (e.g., subdomain matrices in PCASM) if > nonzerostate > >> is up to date. This works if the sparsity pattern never drops nonzeros, > >> but that's no longer true if reset is allowed. I can reset a matrix, > >> preallocate and assemble it so that the global number of nonzeros will > be > >> the same as before the resetting, but local sparsity patterns will > change. > >> This could happen, for example, when I have moving particles or, less > >> exotically, when I have elastic contact and nodes move past each other. > >> That will break PCASM. > > Just increase the nonzerostate flag by one on a reset (that is there is > no reason to ever set it back to zero). Now nonzerostate is monotonically > increasing. > > Barry > > > Barry > > > > > On pretty simple and reliable solution would be to take a cryptographic > > hash of the row/col arrays. I assume BG is really atrocious at hashing, > > but is it so bad that this is not viable? (There are several places > > where we use kinda fragile state counters or trust the user, but hashes > > would make rebuilding more reliable and transparent.) > >
