[Beowulf] MPICH question

[William Gropp]

At 07:30 PM 2/21/2005, John Lau wrote:
>Hi,
>
>I have a question on the number of processes spawned by MPICH. I am
>using MPICH 1.2.5.2 --with-device=ch_p4. When I use mpirun -np 4 to
>start a mpi process, total 8 processes will be spawned. But only 4
>processes have loadings on CPUs. I would like to know if it is the
>correct behavior of MPICH? And what's the use of the 4 no-loading
>process? Thank you.

Those four processes are used to listen for connection requests.  They are 
part of the ch_p4 device, which is built on top of the p4 communication 
layer.  The p4 layer is quite venerable (it may be older than you are), and 
predates the wide use of threads.  There is an option to use a thread 
instead of a process to listen for connection requests, but you best bet is 
to switch to MPICH2, which uses a very different (and more scalable and 
modern) architecture.  Specific questions should be directed to 
mpi-maint at mcs.anl.gov (for MPICH) or mpich2-maint at mcs.anl.gov (for MPICH2).

Bill


>Best regards,
>John Lau
>--
>John Lau Chi Fai
>Cluster Technology Ltd.
>cflau at clustertech.com
>Tel: (852) 2994-3727
>Fax: (852) 2994-2101
>
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William Gropp
http://www.mcs.anl.gov/~gropp