Boosting the performance of NOW-based shared memory multiprocessors through directory hints

Directory hints help a node in a NOW-based shared memory multiprocessor to keep track of where valid copies of a memory block may reside. With this information the node can fetch the block directly from those nodes on a read miss. In this way the number of network transactions to serve the miss may be reduced and the expensive directory lookup operation may be removed from the critical path. We discuss the issues involved in implementing the directory hint scheme on a NOW-based shared memory multiprocessor and examine one such implementation, which employs a small and fast cache to store the hints. Our simulation results show that the directory hint scheme can effectively reduce the read stall time. Also its performance is very competitive compared with a more expensive implementation which uses a large level-three cache. A drawback of the scheme is that it will introduce extra network traffic. We believe that the state-of-the-art interconnection networks, such as those built upon the SCI Spider (Galles, 1996) and the Intel Cavallino (Carbonaro and Verhoorn, 1996) chips, provide the opportunity to make the directory hint scheme feasible even with the slower network such as the one built by Myrinet switches (N.J.B et al., 1996).