Comparative evaluation of latency reducing and tolerating techniques

Techniques that can cope with the large latency of memory accesses are essential for achieving high processor utilization in large-scale shared-memory multiprocessors. In this paper, we consider four architectural techniques that address the latency problem: (i) hardware coherent caches, (ii) relaxed memory consistency, (iii) softwarecontrolled prefetching, and (iv) multiple-context support. While some studies of benefits of the individual techniques have been done, no study evaluates all of the techniques within a consistent framework. This paper attempts to remedy this by providing a comprehensive evaluation of the benefits of the four techniques, both individually and in combinations, using a consistent set of architectural assumptions. The results in this paper have been obtained using detailed simulations of a large-scale shared-memory multiprocessor. Our results show that caches and relaxed consistency uniformly improve performance. The improvements due to prefetching and multiple contexts are sizeable, but are much more applicationdependent. Combinations of the various techniques generally attain better performance than each one on its own. Overall, we show that using suitable combinations of the techniques, performance can be improved by 4 to 7 times.