Update XIOS depth calc for Genoa

[Oakley Brunt (oakleybrunt)]

PR Summary

Sci/Tech Reviewer: Hacka Fett (@christophermaynard)
Code Reviewer:

This ticket addresses issue #601 - XIOS server distribution is not currently optimised for Genoa nodes. Changes here are limited to the launch-exe script, specifically updating the calculation of XIOS_DEPTH to use CORES_PER_NODE instead of hard-coded values for Milan architecture.

The implementation uses calculations for most depth settings, up to CORES_PER_NODE / 16. This would result in 12-cpu gaps between MPI ranks on Genoa hardware and 8-cpu gaps on Milan. Beyond this, depths are hard-coded to reduce from 8 by a factor of 2 (8, 4, 2). The reason for this is illustrated here:

A depth of 8 is more optimal for Genoa nodes than a depth of CORES_PER_NODE / 32, which was the next step in the original sequence. This would give a depth of 6 which would not fit as neatly into cache boundaries as a depth of 8.

As shown in the Genoa hardware diagram in the EX User Guide, a NUMA region is shared by 24 cpus and L3 cache is shared by 8 cpus in a block. This means that a depth of 12 is slightly less optimal than a depth of 8 since memory operations will have to pass via the NUMA region. However, this is a trade-off for more complete utilisation of IO server nodes.

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trac.log

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