Hyper-threading activated on Euler

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After successfully testing Intel Hyper-Threading (HT) on a subset of Euler nodes, we have decided to activate this feature on the whole cluster. HT has already been enabled on all new Euler III nodes since the start of the beta testing phase. We are now progressively activating it on all Euler I and II nodes as part of the periodic reinstallation of all compute nodes in the cluster. This process will take a couple of weeks.

The operating system will now see 48 logical cores on a 24-core node. The batch system will also see these logical cores, but will continue to use physical cores when allocating resources to batch jobs. A job requesting 1 (physical) core will thus get two logical cores and will be able to execute two threads simultaneously — if the application supports it.

This change is completely transparent. You should continue to submit your jobs as before, you do not need to change anything to your job submission scripts.

If you want to run performance tests during the transition phase, you can use the bsub options below to explicitly request nodes with/without HT:

bsub -R "select[nthreads==2]" ...     # request nodes where HT is enabled (2 threads per core)
bsub -R "select[nthreads==1]" ...     # request nodes where HT is NOT enabled (1 thread per core)

Based on our tests, HT gives a (sometimes substantial) performance boost to applications that support it, and has no negative impact on applications that don't. If you notice anything unusual, please inform us.

Refer to the Using hyperthreading page for further details.