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Effects of Contention on Message Latencies in Large Supercomputers

Abhinav Bhatele  (University of Illinois at Urbana-Champaign)
Laxmikant V. Kale  (University of Illinois at Urbana-Champaign)
ACM Student Competition Session
Wednesday,  10:30AM - 10:48AM
Room 17A/17B
Significant research was done on interconnect topologies and topology-aware mapping for parallel computers in the 80s. With the deployment of virtual cut-through, wormhole routing and faster interconnects, message latencies reduced and research in the area died down. This work claims that with the emergence of very large supercomputers, topology effects have resurfaced again. It presents an evaluative study of the dependence of message latencies (in the absence and presence of contention) on the number of hops traversed by messages. This work suggests that in presence of contention, the effect of hops on latencies is quite significant and increases with the diameter of the interconnect. These claims are supported by benchmark studies for four supercomputers -- Watson Blue Gene/L, ANL's Blue Gene/P, PSC's XT3 and ORNL's XT4. The poster will also show significant improvements in performance for a real quantum chemistry application called OpenAtom with the use of topology-aware mapping schemes.
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