[petsc-dev] Bug introduced in MatGetSubmatrices()

[Barry Smith]

> On Jan 20, 2017, at 7:01 PM, Matthew Knepley <knepley at gmail.com> wrote:
> 
> On Fri, Jan 20, 2017 at 3:08 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:
> 
> > On Jan 20, 2017, at 3:01 PM, Zhang, Hong <hzhang at mcs.anl.gov> wrote:
> >
> > We set C->submat_singleis = false as default.
> > User turns it on when he knows a single IS will be used for MatGetSubMatrices(), e.g., when asm is used.
> > User should turn the flag off after it is being used.
> 
>    Right, this is why I am confused why this is messing up Mat since presumably he is not setting it?
> 
> ASM is turning this flag on, and it is corrupting my UNRELATED call to MatGetSubMatrices().

  Yes, but you need to tell us exactly how. The flag is set into C->submat_singleis  which is associated with the C from ASM, so why is it affecting your UNRELATED call to MatGetSubMatrices(). It is not a global variable so shouldn't. 

   Explain or send a code that reproduces the problem.

   Barry

> 
>   Matt
>  
> >
> > This flag is for optimization -- avoid MPI_Allreduce, which is expensive for np>10k.
> > MatGetSubMatrices() works without this flag turns on.
> > I just merged the improved MatGetSubMatrices_MPIAIJ() which only calls MPI_Allreduce once
> > for repeated use. Previously, it calls MPI_Allreduce twice for each call of MatGetSubMatrices_MPIAIJ().
> >
> > Hong
> >
> > ________________________________________
> > From: Barry Smith [bsmith at mcs.anl.gov]
> > Sent: Friday, January 20, 2017 1:11 PM
> > To: Matthew Knepley
> > Cc: Zhang, Hong; PETSc
> > Subject: Re: [petsc-dev] Bug introduced in MatGetSubmatrices()
> >
> >> On Jan 20, 2017, at 12:45 PM, Matthew Knepley <knepley at gmail.com> wrote:
> >>
> >> On Fri, Jan 20, 2017 at 11:55 AM, Barry Smith <bsmith at mcs.anl.gov> wrote:
> >>
> >>> On Jan 20, 2017, at 11:49 AM, Matthew Knepley <knepley at gmail.com> wrote:
> >>>
> >>> On Fri, Jan 20, 2017 at 10:49 AM, Hong <hzhang at mcs.anl.gov> wrote:
> >>> Matt,
> >>> By default, the flag C->submat_singleis = false.
> >>> In PCSetUp_ASM(), we set it as 'true' to use MatGetSubMatrices_MPIAIJ_SingleIS().
> >>>
> >>> Can you check the value of this flag in your case?
> >>>
> >>> The problem is the following:
> >>>
> >>>  1) We use MatGetSubMatrices() to extract small matrices in order to form a preconditioner
> >>>
> >>>  2) We do this at each Newton iteration
> >>>
> >>>  3) We use ASM as a preconditioner for the eventual Newton solve
> >>>
> >>>  4) The second time we call MatGetSubMatrices(), it has this flag set, even though we are using multiple ISes
> >>>
> >>> Solution: ALSO check that the user is in fact passing a single IS.
> >>>
> >>    This requires a communication. I am confused, is the number of IS changing each time? If not why is the flag set?
> >>
> >> ASM sets this flag because it knows that IT is going to call MatGetSubMatrices() later, but it unsafe if any user calls
> >> MatGetSubMatrices() as well. I think overall its a fragile design and should be scrapped.
> >
> >   You mean the user calls the SAME MatGetSubMatrices with the same matrices, right? Not a completely different unrelated MatGetSubMatrices() which should not be affect by the previous unrelated call.
> >
> >
> >   Barry
> >
> >>
> >>  Matt
> >>
> >>
> >>   Barry
> >>
> >>>  Matt
> >>>
> >>> Hong
> >>>
> >>> It comes from here:
> >>>
> >>>  https://bitbucket.org/petsc/petsc/commits/c10200c1442b553b7ad65c70101560db4fa22e78
> >>>
> >>> If we ask for more than 1 matrix, it dispatches to
> >>>
> >>>  MatGetSubMatrices_MPIAIJ_SingleIS()
> >>>
> >>> but then fails here
> >>>
> >>>  https://bitbucket.org/petsc/petsc/annotate/2e559809f9aee9c95ee79eb0939630cfe5502c8d/src/mat/impls/aij/mpi/mpiov.c?at=master&fileviewer=file-view-default#mpiov.c-1306
> >>>
> >>> because ismax > 1. I think the ismax check needs to move up to here
> >>>
> >>>  https://bitbucket.org/petsc/petsc/annotate/2e559809f9aee9c95ee79eb0939630cfe5502c8d/src/mat/impls/aij/mpi/mpiov.c?at=master&fileviewer=file-view-default#mpiov.c-2012
> >>>
> >>> but I don't know for sure. Please fix this since it is breaking PyLith.
> >>>
> >>>  Matt
> >>>
> >>> --
> >>> 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
> >
> >
> 
> 
> 
> 
> -- 
> 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