[Users] Fw: TAT/ Slab Error

[Roland Haas]

Hello Maria,

>  Am I misinterpreting when I read "num-threads is larger than the
> value for cores or procs"?
Yes you are indeed misreading. The example I gave was:

./simfactory/bin/sim submit testrun01 --cores 4 --num-threads 2

ie num-thread=2 and cores (which is a synomym for procs) is 4.

> The cluster I am using (ThornyFlat) gives me the formula:
> procs* num-stm = np*num-threads
> Disregarding simultaneous multithreading (num-stm = 1) and considering np>1, gives procs>num-threads

> What I heard at the last ETK meeting was that procs is nr. processes,
> and it will be divided by the num threads.
No, procs is not the number of processes. It comes from processORs from
the time where "processor" and "processor core" was the same thing.

--procs is the number total number of threads to create. The ends up
being the same as the number of (logical) cores to use, and (ignoring
hyperthreading) the same as the number of physical cores.

See:

http://simfactory.org/info/documentation/userguide/processterminology.html

My advise is usually:

* forget about hyperthreading (at least until you have numbers that
  indicate to you that for your simulation target you actually see a
  benefit)
* use --cores for the total number of physical cores you want
* use --num-threads for the number of threads you want per MPI rank

everything else is complicated / error-prone / dangerous / mostly
untested.

Yours,
Roland

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