[petsc-users] Optimized mode

[Mark Adams]

I am suspicious that he gets the exact same answer with a debug build.

You might try -O2 (and -O1, and -O0, which should be the same as your debug
build).

On Tue, Sep 17, 2019 at 2:21 PM Emmanuel Ayala via petsc-users <
petsc-users at mcs.anl.gov> wrote:

> OK, thanks for the clarification! :D
>
> Regards.
>
> El mar., 17 de sep. de 2019 a la(s) 13:16, Smith, Barry F. (
> bsmith at mcs.anl.gov) escribió:
>
>>
>>   In parallel the order of operations will always change with different
>> number of processes; even with the same number the orders will be changed
>> based on order of arrival of data from other processes; so identical runs
>> can produce different results.
>>
>>    Barry
>>
>>
>> > On Sep 17, 2019, at 12:17 PM, Emmanuel Ayala <juaneah at gmail.com> wrote:
>> >
>> > Hi, thanks for the quick reply.
>> >
>> >
>> > El lun., 16 de sep. de 2019 a la(s) 23:40, Smith, Barry F. (
>> bsmith at mcs.anl.gov) escribió:
>> >
>> >   What do you mean by 2.0e-4 units ?   If you mean the last 4 digits
>> may differ in the two solutions,
>> >
>> > yes, that is the meaning
>> >
>> > yes that is completely normal. How many digits you lose depends on the
>> order of the operations  and the condition number of the matrix and and for
>> elasticity that will very easily be  greater than 10^4
>> >
>> > OK. I understand that two solution can produce slightly different
>> results. So,  I guess, the optimized mode produce slightly different
>> results because internally something change (order of some operations,
>> maybe) when the number of process change?.
>> >
>> >   Barry
>> >
>> > From:
>> https://pdfs.semanticscholar.org/dccf/d6daa35fc9d585de1f927c58cc29c4cd0bab.pdf
>> >
>> > We conclude this section by noting the need for care in interpreting
>> the forward error. Experiments in [24] show that simply changing the order
>> of evaluation of an inner product in the substitution algorithm for
>> solution of a triangular system can change the forward error in the
>> computed solution by orders of magnitude. This means, for example, that it
>> is dangerous to compare different codes or algorithms solely in terms of
>> observed forward errors.
>> >
>> > Thank you very much!
>> > Best regards.
>> >
>> > > On Sep 16, 2019, at 11:13 PM, Emmanuel Ayala via petsc-users <
>> petsc-users at mcs.anl.gov> wrote:
>> > >
>> > > Hi everyone,
>> > >
>> > > I have a code for elastic linear analysis which involves DMDA,
>> SuperLu and matrix operations. I have a MPI vector which contains the nodal
>> displacements, when I run the code in debug mode i get the same value for
>> the norm (2 or Inf), for different number of process. But when I use the
>> optimized mode, I have small variations for the same norm depending on the
>> number of process.
>> > >
>> > > -Debug mode: the same norm value for any number of processes.
>> > > -Optimized mode: the norm value changes with the number of processes.
>> The variations are around 2.0e-4 units.
>> > >
>> > >  This is normal?
>> > >
>> > > For my optimized mode I used the next configuration
>> > >
>> > > ./configure --with-debugging=0 COPTFLAGS='-O3 -march=native
>> -mtune=native' CXXOPTFLAGS='-O3 -march=native -mtune=native' FOPTFLAGS='-O3
>> -march=native -mtune=native' --download-mpich --download-superlu_dist
>> --download-metis --download-parmetis --download-cmake
>> --download-fblaslapack=1 --with-cxx-dialect=C++11
>> > >
>> > > Best regards.
>> >
>>
>>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20190917/6c315632/attachment-0001.html>