[petsc-users] Running the same problem multiple time, but the matrix is not the same

[Khai Pham]

Thanks, Matthew! I will look at numerical stability more carefully.

Khai

On Thu, Dec 8, 2016 at 6:25 PM, Matthew Knepley <knepley at gmail.com> wrote:

> On Thu, Dec 8, 2016 at 5:10 PM, Khai Pham <khaipham at utexas.edu> wrote:
>
>> Hi Hong,
>>
>> I more concern about the difference between A1 and A2 is in the order of
>> O(1.e-8) as I run the same code twice. Should it be in the order of machine
>> epsilon?
>>
>
> If you have 17 orders of magnitude difference between elements, then its
> easy to cancellation when doing subtraction and have
> differences on that order due to permuted operations.
>
>   Matt
>
>
>> Khai
>>
>> On Thu, Dec 8, 2016 at 4:54 PM, Hong <hzhang at mcs.anl.gov> wrote:
>>
>>> Khai :
>>>>
>>>> Thanks for your response. The output is not the solution. They are the
>>>> component of the matrix in matlab format. I would expect the difference in
>>>> the order of machine precesion. The difference in solutions in two runs is
>>>> in the order of 1e-5.
>>>>
>>>
>>> If your matrices A1 and A2 have difference of O(1.e-8), then the
>>> computed solution may differ by
>>> Condition_number(A) * machine_epsion. Do you know cond(A)?
>>>
>>> Please alway send your request to petsc-maint.
>>>
>>> Hong
>>>
>>>>
>>>>
>>>> On Dec 8, 2016 4:01 PM, "Hong" <hzhang at mcs.anl.gov> wrote:
>>>>
>>>> Khai :
>>>> Your solution components have values ranging from 1.e+9 to 1.e-8, and
>>>> the values only differ in the order of 1.e-8, which are within
>>>> computational error tolerance.
>>>> I would consider all solutions same within the approximation tolerance.
>>>>
>>>> Hong
>>>>
>>>> Hello,
>>>>>
>>>>> I have problem with matrix assembly for linear solver KSP. I run the
>>>>> the same problem with 4 processors multiple time. Using flag
>>>>> -mat_view ::ascii_matlab to view the matrix. The matrix outputs are not the
>>>>> same during each run. Please see the attached file for the comparison
>>>>> between two runs. I checked all the input for each processor as it calls to
>>>>> MatSetValues ( indices of the matrix and values ) and it's consistent all
>>>>> the time. I also checked the allocation information (with flag -info) and
>>>>> it looks fine. Could you give me an advice how to deal with this issue?
>>>>> Thanks !
>>>>>
>>>>> Best,
>>>>> Khai
>>>>>
>>>>
>>>>
>>>>
>>>
>>
>
>
> --
> 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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