[petsc-dev] What should MatReset() do?

[Barry Smith]

> On Mar 6, 2015, at 8:22 PM, Dmitry Karpeyev <karpeev at mcs.anl.gov> wrote:
> 
> 
> 
> On Fri, Mar 6, 2015 at 6:03 PM Barry Smith <bsmith at mcs.anl.gov> wrote:
> 
> > On Mar 6, 2015, at 4:41 PM, Jed Brown <jed at jedbrown.org> wrote:
> >
> > Dmitry Karpeyev <karpeev at mcs.anl.gov> 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;

  This is ok. So long as the two sub matrix states are always increasing the state of the total matrix will increase.

>     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.

  They should not be "blown away". They should also be reset and in being reset their nonzerostate will never get smaller.


>  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?

   Why are you reseting the nonzerostate to zero, just don't do that. 
> 
> 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.

  It is possible that somewhere the state is not properly handled by being incremented.

  Barry

> 
> >> 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.)
>