[Pw_forum] How to tune cputimes using mpich2-1.0.8

[Mahmoud Payami Shabestari]

Dear Axel,


> mahmoud,
> 
> as i already wrote to you in private e-mail. the huge discrepancy
> between wall and cpu time casts a severe doubt on the usefulness
> of these numbers. 
> 
> as i was stating in the previous mail. if the MPI library takes
> a significant time of the total time, there is a problem elsewhere
> that needs to be investigated first. you are obviously severely
> overloading your machine. 
> 
> MP> These are the results of an scf calculation for 21-layered Al(110)
> slab
> MP> with Ecut=22Ry, k_mesh=45x45x1, degauss=0.05 on a box with 2 x
> amd64 quad
> MP> (8 cores).
> MP> The memory used in mpich2 is less than half of that in (default
> MP> switches)openmpi.
> 
> how did you determine that?
> 
Using the "top" command and the fact that compiling with "shared" switch
causes just one copy of lib be present in memory while without that the
number of copies is the same as the number of processes.

Bests,
       mahmoud

 
>

 you have to be careful to differentiate between memory
> being mapped into address space, physical memory used 
> and virtual memory being in use. the respective columns
> in displayed in, e.g. top are: VIRT  RES and CODE+DATA)
> 
> cheers,
>    axel.
> 
> 
> MP> 
> MP> Cheers,
> MP>        mahmoud
> MP> 
> MP> 
> MP> >
> MP> > in general, it is not a good idea to try an play with
> MP> > compiler optimization in the hope to make an MPI implementation
> MP> > (and thus communication) faster.
> MP> >
> MP> > if your application performance would depend so much on the
> MP> > optimization level use to compile your MPI library, then either
> MP> > you have a very crappy MPI implementation, or your application
> MP> > spends too much time in MPI calls. in the latter case, that
> MP> > usually corresponds to severe overload of your communication
> MP> > hardware and taking care of that would give you a much better
> MP> > performance increase than any compiler switches will.
> MP> >
> MP> > basically for almost _any_ system library (and that includes
> MP> > ATLAS and FFTW, btw) it is best to stick to a moderate
> optimization
> MP> > (-O) as aggressive optimization may interfere with the
> implemented
> MP> > algorithms and existing optimizations (e.g. both ATLAS and FFTW
> MP> > include optimizations on the C-language and algorithm level,
> higher
> MP> > compiler optimization can change the semantics of your code and
> MP> > thus negate the optimizations performed in the code). even more
> MP> > so, with many current compilers, aggressive optimization (-O3 or
> MP> > higher) incures a very high risk of the compiler miscompiling
> MP> > your library and thus leading to uncontrollable crashes or wrong
> MP> > results. that doesn't mean, that there may be a measurable
> benefit
> MP> > for singular cases, but from over 10 years of experience in
> MP> > management of HPC resources the overall effect is problematic.
> MP> > most of the time it is hard enough to chase down application bugs
> MP> > that are real or cause by compilers, you don't want to add having
> MP> > to track down problems in your libraries to that.
> MP> >
> MP> > cheers,
> MP> >    axel.
> MP> >
> MP> >
> MP> > MP> Any comment is highly appreciated.
> MP> > MP>
> MP> > MP> Best regards,
> MP> > MP> Mahmoud Payami
> MP> > MP> Phys. Group,
> MP> > MP> Atomic Energy Org. of Iran  
> MP> > MP>
> MP> > MP> _______________________________________________
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> MP> > MP>
> MP> >
> MP> > --
> MP> >
> =======================================================================
> MP> > Axel Kohlmeyer   akohlmey at cmm.chem.upenn.edu  
> http://www.cmm.upenn.edu
> MP> >    Center for Molecular Modeling   --   University of
> Pennsylvania
> MP> > Department of Chemistry, 231 S.34th Street, Philadelphia, PA
> 19104-6323
> MP> > tel: 1-215-898-1582,  fax: 1-215-573-6233,  office-tel:
> 1-215-898-5425
> MP> >
> =======================================================================
> MP> > If you make something idiot-proof, the universe creates a better
> idiot.
> MP> 
> MP> _______________________________________________
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> MP> 
> 
> -- 
> =======================================================================
> Axel Kohlmeyer   akohlmey at cmm.chem.upenn.edu   http://www.cmm.upenn.edu
>    Center for Molecular Modeling   --   University of Pennsylvania
> Department of Chemistry, 231 S.34th Street, Philadelphia, PA 19104-6323
> tel: 1-215-898-1582,  fax: 1-215-573-6233,  office-tel: 1-215-898-5425
> =======================================================================
> If you make something idiot-proof, the universe creates a better idiot.
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