Influence of cross-interferences on blocked loops

Abstract

State-of-the art data locality optimizing algorithms are targeted for local memories rather than for cache memories. Recent work on cache interferences seems to indicate that these phenomena can severely affect blocked algorithms cache performance. Because of cache conflicts, it is not possible to know the precise gain brought by blocking. It is even difficult to determine for which problem sizes blocking is useful. Computing the actual optimal block size is difficult because cache conflicts are highly irregular. In this article, we illustrate the issue of precisely evaluating cross-interferences in blocked loops with blocked matrix-vector multiply. Most significant interference phenomena are captured because unusual parameters such as array base addresses are being considered. The techniques used allow us to compute the precise improvement due to blocking and the threshold value of problem parameters for which the blocked loop should be preferred. It is also possible to derive an expression of the optimal block size as a function of problem parameters. Finally, it is shown that a precise rather than an approximate evaluation of cache conflicts is sometimes necessary to obtain near-optimal performance.