Special Issue Paper

Computer Science - Research and Development

, Volume 28, Issue 2, pp 147-155

First online:

The design of ultra scalable MPI collective communication on the K computer

  • Tomoya AdachiAffiliated withFujitsu Limited Email author 
  • , Naoyuki ShidaAffiliated withFujitsu Limited
  • , Kenichi MiuraAffiliated withFujitsu Limited
  • , Shinji SumimotoAffiliated withFujitsu Limited
  • , Atsuya UnoAffiliated withRIKEN
  • , Motoyoshi KurokawaAffiliated withRIKEN
  • , Fumiyoshi ShojiAffiliated withRIKEN
  • , Mitsuo YokokawaAffiliated withRIKEN

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This paper proposes the design of ultra scalable MPI collective communication for the K computer, which consists of 82,944 computing nodes and is the world’s first system over 10 PFLOPS. The nodes are connected by a Tofu interconnect that introduces six dimensional mesh/torus topology. Existing MPI libraries, however, perform poorly on such a direct network system since they assume typical cluster environments. Thus, we design collective algorithms optimized for the K computer.

On the design of the algorithms, we place importance on collision-freeness for long messages and low latency for short messages. The long-message algorithms use multiple RDMA network interfaces and consist of neighbor communication in order to gain high bandwidth and avoid message collisions. On the other hand, the short-message algorithms are designed to reduce software overhead, which comes from the number of relaying nodes. The evaluation results on up to 55,296 nodes of the K computer show the new implementation outperforms the existing one for long messages by a factor of 4 to 11 times. It also shows the short-message algorithms complement the long-message ones.


K computer MPI collective communication Torus network