I try a couple of things including your suggestion. I also find out this has been reported before,
but there seems to be no clear solution so far:
Here is what I observe:
I keep the problem size fixed with 24 processes. I use two nodes with 8-core each and 2-core each.
1. When it is oversubscribed (12 process/processor), sys vs. user time is much higher than less-subscribed (1.5 process/processor).
Almost The wall clock does not improve too much :-(
2. I try following options, individually and collectively, no difference
mpirun --mpi_yield_when_idle 1 --mca btl tcp,sm,self --mca coll_hierarch_priority 100 ...
3. older openmpi version (1.3) seems to be better than new version (1.3.2), but not significantly.
By the way, I am working on Amazon EC2 (VM-host). Will that make any difference?
On Fri, Jun 26, 2009 at 11:28 PM, Ralph Castain <firstname.lastname@example.org>
If you are running fewer processes on your nodes than they have processors, then you can improve performance by adding
-mca mpi_paffinity_alone 1
to your cmd line. This will bind your processes to individual cores, which helps with latency. If your program involves collectives, then you can try setting
-mca coll_hierarch_priority 100
This will activate the hierarchical collectives, which utilize shared memory for messages between procs on the same node.
On Jun 26, 2009, at 9:09 PM, Qiming He wrote:
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
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