[aspect-devel] pressure normalization

[Thomas Geenen]

it seems to have been a glitch in the system.
i reran the experiments and the walltime spend in the
pressure_normalization has reduces a lot
i now see timings for all cores similar to the lowest timings in the
previous run.

for that run i observed large outliers in the walltime for several cores
up to 30 times higher with a lot of time spend in the global mpi routines
MPI::sum etc

next on the list is the call to compress
dealii::BlockMatrixBase<dealii::TrilinosWrappers::SparseMatrix>::compress

cheers
Thomas
ps i was running between 100 and 200K dfd per core for the timing runs


On Fri, Aug 30, 2013 at 8:24 PM, Timo Heister <heister at clemson.edu> wrote:

> > Interesting. The function contains one explicit MPI call, one creation
> > of a completely distributed vector (should not be very expensive) plus
> > one copy from a completely distributed to a ghosted vector (should also
> > not be very expensive). Can you break down which part of the function is
> > expensive?
>
> It might be one of 3 things:
> 1. Thomas found a work imbalance in this function (as he said, some
> processor might not have anything to do). This could show up in his
> instrumentation of processors being idle (but does not mean it takes a
> significant amount of total runtime).
> 2. It is instead a work imbalance/issue in the computation that
> happens before the normalization and the timers are not synchronized
> correctly.
> 3. He has only very few unknowns per processor which skews the timings.
>
> > That
> > said, I always tried to follow the principle of least surprise, which
> > would mean to make sure that the pressure is normalized or that the
> > linear systems are indeed solved to sufficient accuracy.
>
> I agree.
>
> > Instead of globally relaxing tolerances or switch off pressure
> > normalization, how about having a section in the manual in which we list
> > ways to make the code faster if you know what you do? I'll be happy to
> > write this section.
>
> Sounds good. Some more ideas:
> - use optimized mode :-)
> - lower order of temperature/compositional discretization
> - disable postprocessing if not needed
>
> --
> Timo Heister
> http://www.math.clemson.edu/~heister/
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>
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