[slurm-users] Using hyperthreaded processors

[Diego Zuccato]

Il 04/11/20 19:12, Brian Andrus ha scritto:

> One thing you will start finding in HPC is that, by it's goal,
> hyperthreading is usually a poor fit.
Depends on many factors, but our tests confirm it can do much good!

> If you are properly utilizing your cores, your jobs will actually be
> slowed by using hyperthreading.  They are not 'extra' cores, but a
> method of swapping a core to a different workload during an idle cycle.
> The operative word being 'idle'. The goal of HPC is get resource usage
> as close to 100% as possible, so there should be no 'idle' cycles.
IIUC that's wrong.
Multithread cores have two independent execution units, but a single MMU
(and, from memory, a single FPU, but the tests gave different results).
So you can have at most one process per core, but if you have
multithreaded (threads share address space, processes don't) code it
will run at about twice the speed it achieves when running on a single
thread.

Tested with FPU-intensive code on our cluster.

What thrashes performance is trying to run different processes in the
two threads of a core.

Just my $.02

-- 
Diego Zuccato
DIFA - Dip. di Fisica e Astronomia
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