[petsc-users] MatMult() returning different values depending on # of processors?

[Peter Brune]

MatGetSubMatrix() is collective on Mat and ISCreateXXX is collective on the
provided comm, so the logic you have built to call it on one proc at a time
is unnecessary at best and most likely incorrect and likely to produce
strange results.  You can forgo the if statement and loop over processors,
create the ISes on the same comm as x, and then call MatGetSubMatrix() once.

- Peter


On Thu, Jun 26, 2014 at 4:26 PM, Arthur Kurlej <akurlej at gmail.com> wrote:

> I cannot send the original code, but I reproduced the problem in another
> code. I have attached a makefile the code, and the data for the x vector
> and A matrix.
>
> I think the problem may be with my ShortenMatrix function, but it's not
> clear to me what exactly is going wrong and how to fix it. So I would
> appreciate some assistance there.
>
>
> Thanks,
> Arthur
>
>
>
> On Wed, Jun 25, 2014 at 6:24 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:
>
>>
>>    Can you send the code that reproduces this behavior?
>>
>>    Barry
>>
>> On Jun 25, 2014, at 4:37 PM, Arthur Kurlej <akurlej at gmail.com> wrote:
>>
>> > Hi Barry,
>> >
>> > So for the matrix C that I am currently testing (size 162x162), the
>> condition number is roughly 10^4.
>> >
>> > For reference, I'm porting MATLAB code into PETSc, and for one
>> processor, the PETSc b vector is roughly equivalent to the MATLAB b vector.
>> So I know that for one processor, my program is performing as expected.
>> >
>> > I've included examples below of values for b (also of size 162),
>> ranging from indices 131 to 141.
>> >
>> > #processors=1:
>> >                          0
>> >      1.315217173959314e-20
>> >      1.315217173959314e-20
>> >      4.843201487740107e-17
>> >      4.843201487740107e-17
>> >      8.166104700666665e-14
>> >      8.166104700666665e-14
>> >      6.303834267553249e-11
>> >      6.303834267553249e-11
>> >      2.227932688485483e-08
>> >      2.227932688485483e-08
>> >
>> > # processors=2:
>> >      5.480410831461926e-22
>> >      2.892553944350444e-22
>> >      2.892553944350444e-22
>> >      7.524038923310717e-24
>> >      7.524038923214420e-24
>> >     -3.340766769043093e-26
>> >     -7.558372155761972e-27
>> >      5.551561288838557e-25
>> >      5.550551546879874e-25
>> >     -1.579397982093437e-22
>> >      2.655766754178065e-22
>> >
>> > # processors = 4:
>> >      5.480410831461926e-22
>> >      2.892553944351728e-22
>> >      2.892553944351728e-22
>> >      7.524092205125593e-24
>> >      7.524092205125593e-24
>> >     -2.584939414228212e-26
>> >     -2.584939414228212e-26
>> >                          0
>> >                          0
>> >     -1.245940797657998e-23
>> >     -1.245940797657998e-23
>> >
>> > # processors = 8:
>> >      5.480410831461926e-22
>> >      2.892553944023035e-22
>> >      2.892553944023035e-22
>> >      7.524065744581494e-24
>> >      7.524065744581494e-24
>> >     -2.250265175188197e-26
>> >     -2.250265175188197e-26
>> >     -6.543127892265160e-26
>> >     1.544288143499193e-317
>> >      8.788794008375919e-25
>> >      8.788794008375919e-25
>> >
>> >
>> > Thanks,
>> > Arthur
>> >
>> >
>> >
>> > On Wed, Jun 25, 2014 at 4:06 PM, Barry Smith <bsmith at mcs.anl.gov>
>> wrote:
>> >
>> >    How different are the values in b? Can you send back a few examples
>> of the different b’s? Any idea of the condition number of C?
>> >
>> >    Barry
>> >
>> > On Jun 25, 2014, at 3:10 PM, Arthur Kurlej <akurlej at gmail.com> wrote:
>> >
>> > > Hi all,
>> > >
>> > > While running my code, I have found that MatMult() returns different
>> values depending on the number of processors I use (and there is quite the
>> variance in the values).
>> > >
>> > > The setup of my code is as follows (I can go into more
>> depth/background if needed):
>> > > -Generate parallel AIJ matrix of size NxN, denoted as A
>> > > -Retrieve parallel AIJ submatrix from the last N-1 rows&columns from
>> A, denoted as C
>> > > -Generate vector of length N-1, denoted as x
>> > > -Find C*x=b
>> > >
>> > > I have already checked that A, C, and x are all equivalent when ran
>> for any number of processors, it is only the values of vector b that varies.
>> > >
>> > > Does anyone have an idea about what's going on?
>> > >
>> > >
>> > > Thanks,
>> > > Arthur
>> > >
>> >
>> >
>>
>>
>
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