[Beowulf] Naive question: mpi-parallel program in multicore CPUs

Nathan Moore ntmoore at gmail.com
Tue Oct 2 07:20:58 PDT 2007

My understanding is that on a multi-core machine, mpi communication routines
(MPI_SEND, etc) are implemented as memory copy instructions.  Accordingly,
message passing within a multi-core node should be very fast compared to
your present cluster.

That said, It seems like all the performance benchmarks suggest that
dual-core chips have the performances of 1.5-1.7 single core chips, so for
the same number of nodes (defined as a CPU core) you wouldn't see the same

All of this course depends on the structure of the code, memory usage, etc -
these are just scaling estimates on my part.



On 10/2/07, Kwan Wing Keung <hcxckwk at hkucc.hku.hk> wrote:
> This is perhap a naive question.
> 10 years before we started using the SP2, but we later changed to Intel
> based linux beowulf in 2001.  In our University there are quite a no. of
> mpi-based parallel programs running in a 178 node dual-Xeon PC cluster
> that was installed 4 years ago.
> We are now planning to upgrade our cluster in the coming year.  Very
> likely blade servers with multi-core CPUs will be used.  To port these
> mpi-based parallel programs to a multi-core CPU environment, someone
> suggested that OpenMP should be used, such that the programs can be
> converted to a multi-thread version.  Nevertheless it may take time, and
> the users may be reluctant to do so.  Also for some of the installed
> programs, we don't have the source code.
> Another user suggested that we may change slightly on the .machinefile
> before executing the "mpirun" command.
> Suppose we are going to run a 8 mpi-task program on a quad-core cluster,
> then only 2 CPUs should be selected, with the ".machinefile" looks like
> "cpu0 cpu1 cpu0 cpu1 cpu0 cpu1 cpu0 cpu1" created, i.e. 4 mpi-tasks will
> be spooled to CPU0 and 4 mpi-tasks will be spooled to CPU1.  But the REAL
> question will be:
>     Will EACH mpi-task be executed on ONE single core?
>     If not, then could there be any Linux utility program to help?
> I asked this question to one of the potential vendor, and the sales
> suddenly suggested "Well, you can buy VMWARE to create virtual CPUs to do
> so."  Do you think it is logical?
> Thanks in advance.
> W.K. Kwan
> Computer Centre
> University of Hongkong
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Nathan Moore
Assistant Professor, Physics
Winona State University
AIM: nmoorewsu
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