[petsc-dev] petsc-dev very broken

[Matthew Knepley]

On Sat, Jan 21, 2012 at 8:44 PM, Jed Brown <jedbrown at mcs.anl.gov> wrote:

> On Sat, Jan 21, 2012 at 20:37, Barry Smith <bsmith at mcs.anl.gov> wrote:
>
>>   We cannot have MatPreallocated() inside MatSetValues() because
>> MatSetValues() is not collective but MatPreallocated() ->
>>  MatSetUpPreallocation() -> MatMPIAIJSetPreallocation() is collective.
>>
>>   Likely this bug has been around a long time but Jed's changes to
>> preallocation business has exposed it.
>>
>
> At has been there since the beginning of time (revision 0).
>
>
>>
>>   How to fix?
>>
>> 1)       Error out if non preallocated matrix sent to MatSetValues()?
>> Yuck?  Cause this means user must then always call MatPreallocated() or
>> MatSetUp() before ever using it.
>>               MatCreate()
>>               MatSetSizes()
>>               MatSetType()
>>               MatSetUp()   or MatXXXSetPreallocation() or
>> MatSetUpPreallocation()
>>               use
>>
>
> Unfortunately, I think this is the only viable choice.
>
>
>>
>> 2)       Pray every one always calls MatSetValues() on each process if
>> not preallocating and make sure internally we never screw up. Yuck.
>>
>
> I fear that this causes deadlock only when someone strong scales the job
> up to where they get an empty process. That is the kind of bug that is hard
> to find and can burn too many cycles to risk.
>

I think we should try to draw out the complete state machine for Mat. This
of-the-cuff reasoning obviously does not
work when things get complicated.

  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
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