Age-based Packet Arbitration in Large k-ary n-cubes
Abstract
As applications scale to increasingly large processor counts, the interconnection
network is frequently the limiting factor in application performance. In order to
achieve application scalability, the interconnect must maintain high bandwidth
while minimizing variation in packet latency. As the offered load in the network
increases with growing problem sizes and processor counts, so does the expected
maximum packet latency in the network, directly impacting performance of
applications with any synchronized communication. Age-based packet arbitration
reduces the variance in packet latency as well as average latency. This paper
describes the Cray XT router packet aging algorithm which allows globally fair
arbitration by incorporating age in the packet output arbitration. We
describe the parameters of the aging algorithm and how to arrive at appropriate
settings. We show that an efficient aging algorithm reduces both the average packet
latency and the variance in packet latency on communication-intensive benchmarks.
