Cache coherence in large-scale shared-memory multiprocessors

Abstract

Private data caches have not been as effective in reducing the average memory delay in multiprocessors as in uniprocessors due to data spreading among the processors, and due to the cache coherence problem. A wide variety of mechanisms have been proposed for maintaining cache coherence in large-scale shared memory multiprocessors making it difficult to compare their performance and implementation implications. To help the computer architect understand some of the trade-offs involved, this paper surveys current cache coherence mechanisms, and identifies several issues critical to their design. These design issues include: 1) the coherence detection strategy, through which possibly incoherent memory accesses are detected either statically at compile-time, or dynamically at run-time; 2) the coherence enforcement strategy, such as updating or invalidating, that is used to ensure that stale cache entries are never referenced by a processor; 3) how the precision of block sharing information can be changed to trade-off the implementation cost and the performance of the coherence mechanism; and 4) how the cache block size affects the performance of the memory system. Trace-driven simulations are used to compare the performance and implementation impacts of these different issues. In addition, hybrid strategies are presented that can enhance the performance of the multiprocessor memory system by combining several different coherence mechanisms into a single system.