A comparative evaluation of software techniques to hide memory latency

Abstract

Software oriented techniques to hide memory latency in superscalar and superpipelined machines include loop unrolling, software pipelining, and software cache prefetching. Issuing the data fetch request prior to actual need for data allows overlap of accessing with useful computations. Loop unrolling and software pipelining do not necessitate microarchitecture or instruction set architecture changes, whereas software controlled prefetching does. While studies on the benefits of the individual techniques have been done, no study evaluates all of these techniques within a consistent framework. This paper attempts to remedy this by providing a comparative evaluation of the features and benefits of the techniques. Loop, unrolling and static scheduling of loads is seen to produce significant improvement in performance at lower latencies. Software pipelining is observed to be better than software controlled prefetching at lower latencies, but at higher latencies, software prefetching outperforms software pipelining. Aggressive prefetching beyond conditional branches can detrimentally affect performance by increasing the memory bandwidth requirements and bus traffic. >