[gmx-developers] max OpenMP threads per rank

Roland Schulz roland at utk.edu
Mon Aug 10 19:22:52 CEST 2015

On Mon, Aug 10, 2015 at 1:06 PM, Szilárd Páll <pall.szilard at gmail.com>

> Hi,
> I ran into the issue that I'm not able to use the number of threads I want
> to on Power8. The high-end chips have 12 cores with 8-way SMT (96 threads)
> and I wanted to do a multi-threaded scaling test (without DD) on 1-2
> sockets. However, it turns out that it is not possible to run on >32
> threads with non-multiples on 32. This is a rather inconvenient limitation
> on any hardware where the hw thread count is not multiple of 32, e.g. on
> the Powe8 chip I'd like to run k*12, k={1..8} threads.
> AFAIK the same issues will arise on KNC/L too.
> Does this limitation originate from the fact that the force reduction is
> implemented in a simple manner (power of two binary tree reduce?). If
> that's the case, should we try to extend this?

No you can run with any number of threads. Only the limit has to be
multiples of 32. Thus to run with 96 you would set the limit to 128.


> Cheers,
> --
> Szilárd

ORNL/UT Center for Molecular Biophysics cmb.ornl.gov
865-241-1537, ORNL PO BOX 2008 MS6309
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