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OpenFOAM machine loads for parallel runs

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Old   May 27, 2009, 05:45
Default OpenFOAM machine loads for parallel runs
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Rishi .
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Hello All,

I have observed that when I use multiple CPUs of the same node is the machine load is quite high (higher than number of proc). In comparison, CFX usually does not behave like this. I observed this pattern with different OF solvers: simpleF, turbF, dieselF, etc.

-When running a job with 4 processes on a Quadcore machine, the load it higher than 4 (~5).
-Even on dual cores load is typically higher ~2.7
(all AMD Opteron machines & used exclusively for OF)

When I use a different (master) node to start a run, then slave nodes have "normal loads = num of Proc".

To avoid this overloading issue, I am using one CPU for mpirun (i.e. Running with #CPU-1 --- 3 proc on quadcore)

It seems that mpirun is responsible for this extra load. Does this mean that OF is not using the CPUs in optimum manner? My guess: when load is more than 100% it is not optimum.

Any comments?
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Old   May 27, 2009, 17:30
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Anton Kidess
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A load average slightly higher than one is not necessary very bad. I think a better metric is CPU time vs. walltime. If walltime is not roughly equal to (CPU time / NCPUs), then your parallelization is not very good.
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