OLCF User Call: Thread Affinity on Rhea and Eos

The OLCF will be resuming the monthly User Conference Call. These calls, which will be held at 12:30 PM Eastern Time on the first Tuesday of each month, are your opportunity to speak with center personnel to get the latest updates, express any concerns you may have, etc.

Monthly Topic: Thread Affinity on Rhea and Eos
We will give an overview of thread affinity on Eos and Rhea and discuss common pitfalls that affect thread layout.
Speaker: Robert French of OLCF/UAO
Slides: Thread Affinity,OLCF-News6-3-14

Summary
We had several good questions.
1. One was about core specialization (-r 1) and also using the -j flag to control thread affinity.
A related talk which covers this for Eos and Titan can be found here:aprun usage.

2. Q. What about helper threads? Can they be controlled?
A. Helper threads, are spawned to assist main thread computation by prefetching memory from various shared caches. They do not execute computation. In some cases they can compete with the main thread for cache space and bandwidth. Generally they can not be controlled by the programmer.

3.Q. Can the -cc numa_node be the default at runtime?
A. -cc numa_node assigns ranks to a particular numa node. We can not make this the default because it would not work for all codes.
For more information on Numa nodes see:
Titan Task Layout
Eos Task Layout

4. Q. Is there a better method to detect the thread architecture automatically at run time?
A. We don’t have one at this time.

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