[petsc-dev] Error with ML preconditioner

[Mark F. Adams]

On Feb 18, 2013, at 4:00 PM, Jed Brown <jedbrown at mcs.anl.gov> wrote:

> Hmm, this debugging check was added by Mark here.
> 
> https://bitbucket.org/petsc/petsc-dev/commits/fb7e4343253b73d3fb40857991a8c6d5d0c7c56b#Lsrc/mat/impls/aij/mpi/mmaij.cT126
> 

The logic here is messed up.  Valid input could result in a non-collectiv result.  So this DEBUG code should be a real check here.  I think the logic is OK to detect a blocked garray locally … but some processors could be proper and others not as Jed pointed out.

> Barry touched the code a bit earlier to add this case:
> 
> https://bitbucket.org/petsc/petsc-dev/commits/c997baff1558fd165412be034e812bb69f90972a
> 
> Prior to that, Mark had reversed the meaning of that bit of code:
> 
> https://bitbucket.org/petsc/petsc-dev/commits/a85625d104df12863d7d1735f9fdbe9b281ef7d2#chg-src/mat/impls/aij/mpi/mmaij.c
> 
> Even in Barry's original code, I don't understand why we can expect this check to be producing a collective result. Surely there can be some ranks that see everything evenly divide and others that do not.
> 
> It seems to me that we should either (a) explicitly disallow matrices with a block size to be built when the blocks are not filled or (b) when the user requests blocks, pad blocks so that we can always use an ISBlock.
> 

It looks like we have (a) now in a debug build.  I'm  not wild about (b), this is only used to use blocked IS for efficiency and we have never measure that this is useful.

So I will comment out the whole 'useblockis' business and let Garth move along.  We could do an all reduce to see if everyone is blocked and then, and only then, use th blocked ISs. This would cost an alreduce.  Not sure what would be better, but for now we can just disable this.

> 
> On Mon, Feb 18, 2013 at 2:35 PM, Garth N. Wells <gnw20 at cam.ac.uk> wrote:
> I have some code using ML as a preconditioner, and  it works fines
> with PETSc 3.3 but crashes with PETSc dev. I'm not setting any of the
> ML parameters.  Hopefully someone will recognise the error (below) and
> save me having to bisect the changesets, which is hard because changes
> in PETSc dev make it hard for me to compile against earlier PETSc dev
> revisions.
> 
> Garth
> 
> 
> [0]PETSC ERROR: --------------------- Error Message
> ------------------------------------
> [0]PETSC ERROR: Petsc has generated inconsistent data!
> [0]PETSC ERROR: Use of blocked not consistant (I am usning blocked)!
> [0]PETSC ERROR:
> ------------------------------------------------------------------------
> [0]PETSC ERROR: Petsc Development HG revision:
> 96a09d6a701823d69aede247ed949c1dd4e7c8ff  HG Date: Mon Feb 18 11:08:29
> 2013 -0600
> [0]PETSC ERROR: See docs/changes/index.html for recent updates.
> [0]PETSC ERROR: See docs/faq.html for hints about trouble shooting.
> [0]PETSC ERROR: See docs/index.html for manual pages.
> [0]PETSC ERROR:
> ------------------------------------------------------------------------
> [0]PETSC ERROR: Unknown Name on a arch-linux2-c-debug named .......
> Mon Feb 18 19:25:17 2013
> [0]PETSC ERROR: Libraries linked from /home/garth/local/gcc/petsc-dev/lib
> [0]PETSC ERROR:
> [3]PETSC ERROR: See docs/index.html for manual pages.
> 

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