Modeling the performance of limited pointers directories for cache coherence
Richard Simoni, Mark Horowitz · 1991
Directory-based protocols have been proposed as an efficient means of implementing cache consistency in large-scale shamxlmemory multiprocessors.One class of these protocols utilizes a limited pointers directory, which stores the identities of a small number of caches containing a given block of data.However, the performance potential of these directories in large-scale machines has been speculative at best.In this paper we introduce an analytic model that not only explains the behavior seen in small-scale simulation studies, but also allows us to extrapolate forward to evaluate the efficiency of limited pointers directories in large-scale systems.Our model shows that miss rates inherent to invalidation-based consistency schemes me relatively high (typically 107o to 60Y0) for actively shared da~across a variety of workloads.We find that limited pointers schemes that resort to broadcasting invalidations when the pointers are exhausted perform very poorly in largescale machines, even if there are sufficient pointers most of the time.On the other han~no-broadcast strategies that limit the degree of caching to the number of pointers in an entry have only a modest impact on the cache miss rate and network traffic under a wide range of workloads, including those in which data blocks are actively accessed by a large number of processors.