Modeling the performance of limited pointers directories for cache coherence

Directory-hsed protocols have been proposed as an efficient means of implementing cache consistency in large-scale sharedmemory multiprocessors. One class of these protocols utilizes a limired pointers directory, which Stores the identities of a Small number of caches mntaining 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 consistencyschemes are relatively high (typically 10% to 60%) for actively shared data, 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 pointas most of the time. On the other hand, no-broadcast slrategies 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 traflic under a wide range of workloads. including those in which data blocks are actively accessed by a large number of processors.