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System time overhead in OF

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Old   September 8, 2010, 06:42
Default System time overhead in OF
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Hi all,

I have noticed that on our cluster that openfoam have a very different cpu usage compared to other cfd codes.

In OF a large fraction of the time is spend in system cpu time compared to user cpu time. Other cfd codes we have had our hands on spend almost 100% of the time in user cpu.

I have made a small case based on the angledDuct case with 178000 cells using simpleFoam. The case was constructed so no IO was performed.

In serial more than 30% of the time is spend in system time. Is this an overhead due to the dynamic memory allocation in OF?

simpleFoam_job.jpg

When running the case in parallel (2 nodes) the system time CPU time fraction drops to a few percent which is difficult for me to understand.

simpleFoam_job2.jpg

Can anyone explain the apparent serial overhead?

Best regards,
Ask
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