[Bi-users] Hyper-threading may be turned off

[Kent Engström]

Dear Bi Users,

since we installed the Bi cluster, the Bi compute nodes have been
running with hyper-threading enabled (see
https://en.wikipedia.org/wiki/Hyper-threading)

That means that the 16 physical cores (8 on each CPU) are turned into
32 logical cores.

You have been able to use this in various ways, for example by using
"sbatch --ntasks-per-core=2" to get 32 MPI ranks on a node, or by
using OpenMP or custom solutions to start 32 active processes on a
node. For some programs, using this has been advantageous. For others,
not.

If you have just used plain MPI without the special option above, you
have not been using hyper-threading in your jobs.


In the tech news, you may have seen news recently about yet another
class of speculative execution vulnerabilities in Intel CPUs, called
MDS (Microarchitectural Data Sampling).

One of the recommended mitigations for this is to turn off
hyper-threading. We will be following the situation, and if the risk
from leaving hyper-threading enabled turns out to be too big, we will
have to turn it off.

This is just an advance warning about this, so that if you do rely on
hyper-threading, you can prepare for running without it. We will get
back to you if we have to take this step.

Best Regards,
/ Kent Engström, NSC