[mpich-discuss] MPI how to support the smp machine?

[Gus Correa]

Hi Liaoyin

Besides Dave Goodell's questions,
I wonder if this may be more of an HPL problem
than of an MPICH2 problem.

Here are some questions/suggestions about HPL:

***

1) If you just want to check if MPICH2 is working,
then don't use HPL.  Use something simpler.
The cpi.c and hellow.c programs in the MPICH2 "examples" directory
will tell you if MPICH2 is working properly, and are way much
simpler than HPL to setup and run.

***

2) Note, there may be a typo on the mpirun command line
on your message, it should be "xhpl" not "xhlp" as you wrote,
unless you changed the executable name.

***

3) Are you sure you are running HPL on 2 cores?

If you use Linux, you can submit your HPL job,
then use "top" (and type "1") to see how many
cores are actually running xhpl.

Do you really see 2 cores in action on "top"?

***

4) What are the contents of your HPL.dat parameter file,
when you try to run on 2 cores?

Is is the same that you use for the one core run or is it different?

This may not be news to you, but here it goes just in case:

To run HPL on a given number of cores on your machine,
the product of the values or Ps and Qs in HPL.dat has to be
equal to the N value on your "mpirun -np N"
For example, if you want to use 2 cores (mpirun -np 2),
you could use this on your HPL.dat file:

1 2 Ps
2 1 Qs

Or to use four cores:

1 2 4 Ps
4 2 1 Qs

(Tip: You *must* use a *single blank space* field separator in HPL.dat.)

***

5) Very small problem sizes don't scale well with
the number of processors.

To see speedups when more cores are added,
you need to choose a relatively large value for Ns in HPL.dat.
However, Ns is constrained by how much memory (RAM) your computer has.
A reasonable upper bound to Ns is sqrt(0.8*memory_in_bytes/8).

See item 5) below.

***

6) Finally, if you haven't read it, the HPL TUNING file is a
*must read* for anybody who wants to run the HPL benchmark:

http://www.netlib.org/benchmark/hpl/tuning.html

***

Good luck!

I hope this helps,
Gus Correa
---------------------------------------------------------------------
Gustavo Correa
Lamont-Doherty Earth Observatory - Columbia University
Palisades, NY, 10964-8000 - USA
---------------------------------------------------------------------



Dave Goodell wrote:
> What MPI implementation are you using?  If you are using MPICH2, please 
> make sure that you are using the latest version (1.2.1).
> 
> -Dave
> 
> On Dec 4, 2009, at 3:02 AM, liaoyin wrote:
> 
>> I am runing the  HPL ( A Portable Implementation of the 
>> High-Performance Linpack Benchmark for Distributed-Memory Computers) 
>> to do linpcke test.
>>
>> I use the Atlas to as blas lib.
>>
>> My machine has  one cpu with four core.
>>
>> I run the mpirun -np 1 ./xhlp   the Gflops is 0.4  (one core to run)
>>
>> but  I run the mpirun -np 2  ./xhlp the Gflops is 0.04 (two core to run)
>>
>> why is the two core is slower.
>>
>> _______________________________________________
>> mpich-discuss mailing list
>> mpich-discuss at mcs.anl.gov
>> https://lists.mcs.anl.gov/mailman/listinfo/mpich-discuss
> 
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