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Process affinity: Hop on the bus, Gus!

January 10, 2014 at 5:00 am PST

Today’s blog post is written by Joshua Ladd, Open MPI developer and HPC Algorithms Engineer at Mellanox Technologies.

At some point in the process of pondering this blog post I noticed that my subconscious had, much to my annoyance, registered a snippet of the chorus to Paul Simon’s timeless classic “50 Ways to Leave Your Lover” with my brain’s internal progress thread. Seemingly, endlessly repeating, billions of times over (well, at least ten times over) the catchy hook that offers one, of presumably 50, possible ways to leave one’s lover -- “Hop on the bus, Gus.” Assuming Gus does indeed wish to extricate himself from a passionate predicament, this seems a reasonable suggestion. But, supposing Gus has a really jilted lover; his response to Mr. Simon’s exhortation might be “Just how many hops to that damn bus, Paul?”

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Open MPI: Binding to core by default

December 18, 2013 at 12:55 pm PST

After years of discussion, the upcoming release of Open MPI 1.7.4 will change how processes are laid out (“mapped”) and bound by default.  Here’s the specifics:

  • If the number of processes is <= 2, processes will be mapped by core
  • If the number of processes is > 2, processes will be mapped by socket
  • Processes will be bound to core
  • MPI_COMM_WORLD ranks will be assigned by slot

These are all the default values — they, of course, can be changed by the user via mpirun CLI options, environment variables, etc.

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How many network links do you have for MPI traffic?

July 19, 2013 at 5:00 am PST

If you’re a bargain basement HPC user, you might well scoff at the idea of having more than one network interface for your MPI traffic.

“I’ve got (insert your favorite high bandwidth network name here)! That’s plenty to serve all my cores! Why would I need more than that?”

I can think of (at least) three reasons off the top of my head.

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Latency Analogies (part 2)

April 8, 2013 at 6:25 am PST

In a prior blog post, I talked about latency analogies.  I compared levels of latencies to your home, your neighborhood, a far-away neighborhood, and another city.  I talked about these localities in terms of communication.

Let’s extend that analogy to talk about data locality.

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Latency Analogies

March 31, 2013 at 6:00 am PST

Multiple readers have told me that it is difficult for them to understand and/or visualize the effects of latency on their HPC applications, particularly in modern NUMA (non-uniform memory access) and NUNA (non-uniform network access) environments.

Let’s breaks down the different levels of latency in a typical modern server and network computing environments.

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