An empirical evaluation of two memory-efficient directory methods

This paper presents an empirical evaluation of two memory-efficient directory methods for maintaining coherent caches in large shared memory multiprocessors. Both directory methods are modifications of a scheme proposed by Censier and Feautrier 5 that does not rely on a specific interconnection network and can be readily distributed across interleaved main memory. The schemes considered here overcome the large amount of memory required for tags in the original scheme in two different ways. In the first scheme each main memory block is sectored into sub-blocks for which the large tag overhead is shared. In the second scheme a limited number of large tags are stored in an associative cache and shared among a much larger number of main memory blocks. Simulations show that in terms of access time and network traffic both directory methods provide significant performance improvements over a memory system in which shared-writeable data is not cached. The large block sizes required for the sectored scheme, however, promotes sufficient false sharing that its performance is markedly worse than using a tag cache.