Distributed directory tags

Abstract

This paper concerns a new directory scheme that can be used as an alternative to the current three types of directory schemes; direct mapped, set associative and fully associative. Both direct mapped and fully associative cache directories can be considered as a special case of the set associative directory since in the direct mapped case there are N congruent classes with a set size of 1 while in the fully associative case there are N sets and only 1 congruent class. However, irrespective of which class of directory scheme is used, since memory or cache directories generally provide a many-to-one mapping of a large address space to a small fast storage, a tag field must be associated with each element to identify one of the many addresses which actually map to the current value in the cache element. Thus, there are as many tags as there are total elements in the directory. The Distributed Directory Tag scheme described here uses the direct mapping of the addresses and distribution of the tags over the directory elements and thereby reduces the storage requirement for the directory tags by a significant factor. Our early findings indicate that addresses from application programs follow a particular distribution, with a strong bias towards less frequent change in the higher order bits. It is this observation that is exploited in the scheme described in this paper. The actual savings depend on the particular distribution chosen for an implementation, but in general, a factor of three reduction can be obtained while providing a hit ratio comparable to the other directory schemes. This savings in the tag bits is achieved while still providing set associativity within each congruent class. This scheme is particularly useful when cost/performance tradeoffs are a consideration. Thus, this novel directory design adds another type to the three existing cache directory schemes.