Re: [Starpu-devel] StarPU+SimGrid: FetchingInput computation

[Luka Stanisic <luka.stanisic@inria.fr>]

Hi Mirko,

Indeed, I was wondering if your platform has any GPUs, but as you said 
it is a simple 4 cores machine. Adding more CPUs or GPUs in future 
shouldnt be a problem.

You are right, SimGrid shouldnt include any significant fetching time to 
the simulation since everything is running with shared memory. However, 
appearance of FetchingInput state in the traces is possible, since 
StarPU is passing through many parts of the code. Still, the duration of 
FetchingInput should be negligible.

Could you please share two paje.trace traces (one for real execution and 
one for SimGrid), so I can try to understand better what is happening? 
If the traces are big (>100MB), it might be better to run your 
application with smaller problem size (if possible).


Also from what I have seen, you are using STARPU_REGRESSION_BASED or 
STARPU_NL_REGRESSION_BASED performance models for your codelets, right? 
Is this something that you need for your application? Personally, I have 
never tried to simulate applications using these models, although I dont 
see any reason why it shouldnt work. The starpu_perfmodel::size_base 
field is actually used by these models, more information is available 
here: http://starpu.gforge.inria.fr/doc/html/OnlinePerformanceTools.html


So my first guess is that you are somehow using codelet perfmodels and 
their size_base incorrectly (or there is an unknown bug in StarPU or 
StarPU+SimGrid code), which makes simulation longer than expected. Then, 
in the traces this is manifested as long FetchingInputs, although 
fetching inputs have nothing to do with the actual problem.


Best regards,
Luka
On 13/12/2016 14:34, Mirko Myllykoski wrote:
> Hi Luka,
>
> and thank you for your reply.
>
> I performed the same experiment twice, once with the size_base field 
> included and once without it. I erased the samples directory before 
> each experiment and gave it a few rounds to calibrate properly 
> (STARPU_CALIBRATE=1). Here are the corresponding sample folders:
>
> https://dl.dropboxusercontent.com/u/1521774/sampling_with_size_base.tar.gz 
>
> https://dl.dropboxusercontent.com/u/1521774/sampling_without_size_base.tar.gz 
>
>
> In this case, the error seems to be about 35%.
>
> As I mentioned in my previous email, the code is shared memory only 
> (at the moment). I performed the experiment on my local machine 
> (quad-core i5) but my plan is to move on to a bigger machine (28 or 42 
> cores per node) and distributed memory once everything works.
>
> I don't quite understand why SimGrid would include any fetching time 
> to the simulation since everything is running in shared memory.
>
> Best Regards,
> Mirko
>
> On 2016-12-12 18:21, Luka Stanisic wrote:
> > Hello Mirko,
> >
> > Indeed, 50% prediction error is quite big and it suggests that
> > something is probably not correctly configured. Could you please send
> > us a compressed version of you ".starpu/sampling" folder, the one from
> > which simulation will read the performance models. This can help us
> > get the first ideas of the machine and application you are trying to
> > simulate.
> >
> > To answer your question, the fetching time is computed based on the
> > size of the data being transfered, latency and bandwidth of the link
> > (in machine.platform.xml file) and the possible contention due to
> > other transfers occurring in parallel.
> >
> > Best regards,
> > Luka
> >
> > On 07/12/2016 12:50, Mirko Myllykoski wrote:
> > > Hi,
> > >
> > > my name is Mirko Myllykoski and I work as a PostDoc researcher for 
> > > the NLAFET project at Umeå University.
> > >
> > > I am currently implementing a (shared memory) numerical software 
> > > using StarPU and I am trying to simulate my code using SimGrid. 
> > > However, I noticed that the simulated execution time is way off 
> > > (about 50%). I checked the generated FxT traces using vite and it 
> > > seems that SimGrid introduces too much fetching time (State: 
> > > FetchingInput) to the simulation.
> > >
> > > How is this fetching time being computed? My performance models 
> > > include the starpu_perfmodel::size_base data field and I guess that 
> > > information is somehow used to compute the fetch time.
> > >
> > > Best Regards,
> > > Mirko Myllykoski
> > > _______________________________________________
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