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- Choose a total number of tasks that is evenly divisible by the number of cores/node for your machine (e.g. asking for 14 total cores on a machine with 12 cores/node is dumb because you will be charged for 24 cores and 10 of them will sit idle).
- Try to make the number of assigned task for each component evenly divisible by the number of cores/node for your machine. It is more efficient to have entire nodes doing the same thing than having some cores on a node doing one thing and other cores doing something else.
For single component (e.g CORE-forced ocean, AMIP atmos) compsets, you might as well use a serial configuration because only the active component will consume appreciable time.
- For the atmosphere:
- Choose NTASKS_ATM so it evenly divides the number of spectral elements in your atmos grid. The number of elements is "nelem" from the latlon grid template files available for each grid grid here . The (ACME inputdata server). The number of physics columns is 9*nelem+2. It is possible to use uneven numbers of elements per MPI task or to use more tasks then there are elements. Doing so will speed up the physics but will not speed up the dynamics so it is less efficient.
- For linux clusters and low numbers of nodes (less than 1000) it is typically best to use NTHREADS_ATM=1. On Titan, Mira and KNL systems, threads should be used. On Edison, sometimes small gains can be achieved by turning on hyperthreading and using 2 threads per MPI task, 24 MPI tasks per node.
- When using threads, there are several additional considerations. The number of MPI tasks times the number of threads per MPI task should be equal to the number of cores on the node (except when using hyperthreading on NERSC). The physics can make use of up to NTASKS_ATM*NTHREADS_ATM = # physics columns. The dynamics by default can only make use of NTASKS_ATM*NTHREADS_ATM = nelem (extra threads are ok, they will just not improve dynamics performance). The new "nested openMP" feature can be used to allow the dynamics to use more threads but this compile time option is not yet enabled by default.
- The table below shows the # elements and acceptable core counts for ACME atm resolutions:
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