[petsc-users] petsc example with known scalability

[Matthew Knepley]

On Fri, May 18, 2012 at 7:06 PM, Mohammad Mirzadeh <mirzadeh at gmail.com>wrote:

> Hi guys,
>
> I'm trying to generate scalability plots for my code and do profiling and
> fine tuning. In doing so I have noticed that some of the factors affecting
> my results are sort of subtle. For example, I figured, the other day, that
> using all of the cores on a single node is somewhat (50-60%) slower when
> compared to using only half of the cores which I suspect is due to memory
> bandwidth and/or other hardware-related issues.
>
> So I thought to ask and see if there is any example in petsc that has been
> tested for scalability and has been documented? Basically I want to use
> this test example as a benchmark to compare my results with. My own test
> code is currently a linear Poisson solver on an adaptive quadtree grid and
> involves non-trivial geometry (well basically a circle for the boundary but
> still not a simple box).
>

Unfortunately, I do not even know what that means. We can't guarantee a
certain level of performance because it not
only depends on the hardware, but how you use it (as evident in your case).
In a perfect world, we would have an abstract
model of the computation (available for MatMult) and your machine (not
available anywhere) and we would automatically
work out the consequences and tell you what to expect. Instead today, we
tell you to look at a few key indicators like the
MatMult event, to see what is going on. When MatMult stops scaling, you
have run out of bandwidth.

    Matt


> Thanks,
> Mohammad
>



-- 
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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