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 sectoredscheme, however, promotes sufficient false sharing that its performance is markedly worse than using a tag cache.
本文介绍了两种用于在大型共享内存多处理器中维护相干缓存的内存有效目录方法的经验评估。两种目录方法都是Censier和Feautrier [5]提出的方案的修改,该方案不依赖于特定的互连网络,并且可以很容易地分布在交错的主存储器中。这里考虑的方案以两种不同的方式克服了原始方案中标签所需的大量内存。在第一种方案中,每个主存储块被分成子块,为其共享大标签开销。在第二种方案中,将有限数量的大标签存储在关联缓存中,并在大量的主存储块之间共享。仿真表明,就访问时间和网络流量而言,这两种目录方法都比不缓存共享可写数据的存储系统具有显着的性能提升。但是,扇区化方案所需的大块大小会促进充分的错误共享,因此其性能明显比使用标记缓存差。 P>
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