Reliability Modeling of SCI Ring-Based Topologies

Performance evaluation and reliability prediction are two important factors in the study of multiprocessor and cluster interconnects. One such interconnect is the Scalable Coherent Interface (SCI). SCI is a point-to-point, ringbased interconnect that can be configured in various switched-ring topologies such as counter-rotating rings and tori. While performance analyses of SCI-based interconnects have been discussed in the literature, reliability evaluation has not received much attention. In addition, the reliability of SCI interconnects configured in many of today’s popular topologies cannot be deduced from earlier work on network reliability as link failures within an SCI interconnect are not independent of one another. A single link failure within the topology results in the failure of the entire ringlet to which the link belongs. This paper presents the results of a reliability study on 1D and 2D k-ary ncube switching fabrics for the Scalable Coherent Interface based on ring elimination rather than link elimination. The study is conducted using reliability models created in UltraSAN, a tool based on Stochastic Activity Networks. The models are verified using both combinatorial and Markov modeling. The results demonstrate the inherent reliability characteristics of a single-ring system can be greatly enhanced by the addition of a second redundant ring. By contrast, the results show that the reliability of a torus does not increase significantly with the addition of redundant rings. Hence, the cost of adding redundant rings to certain topologies may or may not be justified, depending upon the degree of reliability sought.