[gmx-developers] max OpenMP threads per rank

[Roland Schulz]

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

> 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.

Roland


>
> Cheers,
> --
> Szilárd
>



-- 
ORNL/UT Center for Molecular Biophysics cmb.ornl.gov
865-241-1537, ORNL PO BOX 2008 MS6309
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://maillist.sys.kth.se/pipermail/gromacs.org_gmx-developers/attachments/20150810/8f0905a1/attachment.html>