[gmx-developers] max OpenMP threads per rank

[Szilárd Páll]

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?

Cheers,
--
Szilárd
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://maillist.sys.kth.se/pipermail/gromacs.org_gmx-developers/attachments/20150810/d3aa8593/attachment.html>