Wafer-scale integration and two-level pipelined implementations of systolic arrays

Abstract

This paper addresses two important issues in systolic array designs. How do we provide fault-tolerance in systolic arrays for yield enhancement in wafer-scale integration implementations? And, how do we design efficient systolic arrays with two levels of pipelining? The first level refers to the pipelined organization of the array at the cellular level, and the second refers to the pipelined functional units inside the cells. The fault-tolerant scheme we propose replaces defective cells with clocked delays. This has the distinct characteristic that data can flow through the array with faulty cells at the original clock speed. We will show that both the defective cells under this fault-tolerant scheme and the second level pipeline-stages can simply be modeled as additional delays in the data paths of "generic" systolic designs. We introduce the mathematical notion of a cut to solve the problem of how to allow for these extra delays while preserving the correctness of the original systolic array designs. The results obtained by applying the techniques described in this paper are encouraging. When applied to systolic arrays without feedback cycles, the arrays can tolerate large numbers of failures (with the addition of very little hardware) while maintaining the original throughput. Furthermore, all of the pipeline stages in the cells can be kept fully utilized through the addition of a small number of delay registers. However, adding delays to systolic arrays with cycles typically induces a significant decrease in throughput. In response to this, we have derived a new class of systolic algorithms in which the data cycle around a ring of processing cells. The systolic ring architecture has the property that its performance degrades gracefully as cells fail. Using our cut theory and ring architectures for arrays with feedback, we have effective fault-tolerant and two-level pipelining schemes for most systolic arrays. As a side-effect of developing the ring architecture approach we have derived several new systolic algorithms. These algorithms generally require only one-third to one-half of the number of cells used in previous designs to achieve the same throughput. The new systolic algorithms include ones for LU-decomposition, QR-decomposition and the solution of triangular linear systems.