Correctness of a Directory-Based Cache Coherence Protocol: Early Experience

Cache coherence protocols of increasing complexities call for automated verification tools which are both efficient and reliable. Most current approaches can only verify protocols at a high level of abstraction, and the model size is limited to a small number of interacting processes. Using a simple full-map directory scheme as example, we show that the verification of a simple protocol becomes overwhelmingly complex when implementation details are taken into account. One way to deal with the complexity is to impose conservative handshaking rules such as acknowledging every single message between caches and memory. Such a conservative approach slows down every transaction in order to avoid race conditions, which are relatively rare. The other approach explored in this paper is to apply verification techniques to the protocol without acknowledgements in order to determine the minimum set of messages needed for correctness. A new verification technique which is extremely efficient and is independent of the model size is applied and several non-obvious problems affecting the correctness of a protocol design are identified by the verification procedure.