CPU affinity issues on a big cluster

[alex-petrenko]

@edbeeching I will leave this here so we don't forget.

Notes:

• cpu_affinity() returns only half of the cores (40 instead of 80) which leads to workers only using 50% of the cores.
• possibly related to the fact that this machine has 2 physical processors? Or maybe custom linux build?

Possible solutions:

• ignore cpu_affinity() and use some other mechanism to retrieve the number of cores.

[edbeeching]

This issue was caused by slurm jobs being launched with --hint=nomultithread , switching this to --hint=multithread solves the problem.