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My simulations don't scale when I use multiple nodes -help

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Old   February 11, 2020, 18:47
Default My simulations don't scale when I use multiple nodes -help
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KTG
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Hey all,


I am having some sort of issue where I am observing a speedup in my simulations as I add more cores within one node, but as soon as I go to two nodes, there is no increase. I can't find any info on this - I tried the -bind-to core mpi flag, but no luck. Has anyone observed this behavior of openFoam on a cluster, or have any ideas about how to fix it? I am using openMPI and SLURM, so I call the solver this way:


mpirun -np $SLURM_NTASKS pimpleFoam -parallel > log


I am using a cluster running centos with these specs:

  • 144 Dual Socket-Six Core Nodes
  • 2.8 GHz Intel Xeon (Westmere X5660) processors
  • RAM:
    • 24 Gbytes memory per node (2 Gbytes per processor core) on most nodes
  • Mellanox QDR Infiniband interconnect
  • Gigabit Ethernet interconnect for management
I was thinking the issue had to do with a bottleneck? Is there a way to check that its actually using the Infiniband ? Any ideas welcome!


Thanks-
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Old   February 12, 2020, 00:09
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HPE
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how many cells - how many cores?
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Old   February 18, 2020, 22:00
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KTG
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Hi,


sorry for the delay! It seems that CFD forums doesn't send me e-mails anymore...



Last time I was playing with it, I used over 5 million cells, with 64 cores vs. 32, and got was appears to be the same result. Before I though it was a bottle neck issue, but for all I know the first node is doing all the work. I also notice that the averaging function object is taking an insanely long time, which may or may not be related.



Thanks for the reply!
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