[gmx-developers] interesting read: exascale w/o threads

[Berk Hess]

Interesting read. The essential message is:
"Thus, we believe that a discussion on threads versus processes boils 
down to “shared everything by default” versus “shared nothing by default”.
We came to the same conclusion in a discussion some time ago. So the 
choice doesn't affect performance, but it strongly affects the code.
For Gromacs I think it's still convenient to have processes+threads, 
since we have many data structures with many small arrays that change at 
domain decomposition time that are needed by all threads in a domain. 
Sharing all these through MPI3 is tedious.

Cheers,

Berk

On 10/08/2015 01:04 PM, Szilárd Páll wrote:
>
> "We believe that portions of the HPC community have adopted the point 
> of view that somehow threads are “necessary” in order to utilize such 
> [manycore/SMP] systems, (1) without fully understanding the 
> alternatives, including MPI 3 functionality, (2) underestimating the 
> difficulty of utilizing threads efficiently, and (3) without 
> appreciating the similarities of threads and processes. This short 
> paper, due to space constraints, focuses exclusively on issue (3) 
> since we feel it has gotten virtually no attention."
>
> http://www.orau.gov/hpcor2015/whitepapers/Exascale_Computing_without_Threads-Barry_Smith.pdf
>
> --
> Szilárd
>
>

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