[Pw_forum] newbie update

[Axel Kohlmeyer]

On Wed, 4 Mar 2009, Carlo Nervi wrote:

CN> Dear all,

dear carlo,

CN> finally I succesfully compiled the code using ifort, mkl
CN> and mpich2.
CN> However, I'm getting odd results, very probably due to
CN> wrong settings.
CN> 
CN> I tried to run the example01 with the following settings:
CN> 
CN> OMP_NUM_THREADS=1
CN> mpiexec -n 8
CN> 
CN> The example is executed, but with a very long time. All 8
CN> CPUs are running
CN> between 5 and 20 % of the cpu time. For some reason we are
CN> not using all the power..

please note, example01 is _very_ small. as a result there is
little data to parallelize over and quite a bit of i/o.

CN> I tried to run also "mpiexec -envall -n 8" (maybe the
CN> OMP_NUM_THREADS is not passed)
CN> but i got identical results.
CN> Here is the results of the first example in the example01
CN> directory:
CN> 
CN> results/si.scf.david.out
CN> 
CN>      Writing output data file silicon.save
CN> 
CN>      PWSCF        :     0.50s CPU time,    7.27s wall time

see. these are _seconds_. unless you run a test that runs _minutes_,
there is little use to these numbers.

as suggested before, a good test to see if your MPI is working well,
are the cp.x inputs in example21. have a look at the readme first.

running them with ./run_example 32 should be sufficient.

CN> On the other hand, if I set
CN> 
CN> OMP_NUM_THREADS=8
CN> and do not use mpiexec I got:
CN> 
CN>      Writing output data file silicon.save
CN> 
CN>      PWSCF        :     7.10s CPU time,    1.12s wall time
CN> 
CN> Therefore results/si.scf.david.out takes 1.12s instead of
CN> 7.27s, a much
CN> more reasonable timing.
CN> Anyone could give me an advice?

this is due to having to create 8x the number of files.

see my comments in the (longish) e-mail from last night with 
benchmarks wrt. to the number of pools to use. it also looks
briefly into the impact of file io. with this small a system,
the impact is correspondingly larger.

cheers,
   axel.

CN> Thank you,
CN>   Carlo
CN> 
CN> --
CN> ------------------------------------------------------
CN> Carlo Nervi carlo.nervi at unito.it Tel:+39 011 6707507/8
CN> Fax: +39 011 6707855 - Dipartimento di Chimica IFM
CN> via P. Giuria 7, 10125 Torino, Italy
CN> http://lem.ch.unito.it/
CN> 
CN> 
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CN> http://www.democritos.it/mailman/listinfo/pw_forum
CN> 

-- 
=======================================================================
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.