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Subject: [OMPI users] CPU user time vs. system time
From: Qiming He (qiming.he_at_[hidden])
Date: 2009-06-26 23:09:48


Hi all,
I am new to OpenMPI, and have an urgent run-time question. I
have openmpi-1.3.2 compiled with Intel Fortran compiler v.11 simply by

./configure --prefix=<my-dir> F77=ifort FC=ifort
then I set my LD_LIBRARY_PATH to include <openmpi-lib> and <intel-lib>
and compile my Fortran program properly. No compilation error.

I run my program on single node. Everything looks ok. However, when I run it
on multiple nodes.
mpirun -np <num> --hostfile <my-hosts> <my-program>
The performance is much worse than a single node with the same size of the
problem to solve (MPICH2 has 50% improvement)

I use top and saidar to find that user time (CPU user) is much lower than
system time (CPU system), i.e,
only small portion of CPU time is used by user application, while the rest
is busy with system.
No wonder I got bad performance. I am assuming "CPU system" is used for MPI
communication.
I notice the total traffic (on eth0) is not that big (~5Mb/sec). What is CPU
system busy for?

Can anyone help? Anything I need to tune?

Thanks in advance

-Qiming