[gmx-developers] max OpenMP threads per rank

Szilárd Páll pall.szilard at gmail.com
Mon Aug 10 19:06:05 CEST 2015


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?

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