VISU: A Simple and Efficient Cache Coherence Protocol Based on Self-updating

Abstract

Existing cache coherence protocols incur high overheads to shared memory systems and significantly reduce the system efficiency. For example, the widely used snooping protocol broadcasts messages at the expense of high network bandwidth overheads, and the directory protocol requires massive storage spaces to keep track of sharers. Furthermore, these coherence protocols have numerous transient states to cover various races, which increase the difficulty of implementation and verification. To mitigate these issues, this paper proposes a simple and efficient, two-state (Valid and Invalid) cache coherence protocol, VISU, for data-race-free programs. We adopt two distinct schemes for the private and shared data to simplify the design. Since the private data does not need to maintain coherence, we apply a simple write-back policy. For shared data, we leverage a write-through policy to make the last-level cache always hold the up-to-date data. A self-updating mechanism is deployed at synchronization points to update stale copies in L1 caches; this obviates the need for the broadcast communication or the directory.

Experimental results show that the VISU protocol achieves a significant reduction (31.0%) in the area overhead and obtains a better performance (2.9%) comparing with the sophisticated MESI directory protocol.