Departmental Papers (CIS)

Date of this Version

April 2004

Document Type

Conference Paper


Copyright 2004 IEEE. Reprinted from Proceedings of the 18th International Parallel and Distributed Processing Symposium 2004 (IPDPS 2004).

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Modern multiprocessors are complex systems that often require years to design and verify. A significant factor is that engineers must allocate a disproportionate share of their effort to ensure that rare corner-case events behave correctly. This paper proposes using "speculation for simplicity" to enable designers to focus on common-case scenarios. Our approach is to speculate that rare events will not occur and rely on an efficient recovery mechanism to undo the effects of mis-speculations.

We illustrate the potential of speculation to simplify multiprocessor design with three examples. First, we simplify the design of a directory cache coherence protocol by speculatively relying on point-to-point ordering of messages in an adaptively routed interconnection network. Second, we simplify a snooping cache coherence protocol by treating a rare coherence state transition as a mis-speculation. Third, we simplify interconnection network design by removing the virtual channels and then recovering from deadlocks when they occur.

Experiments with full-system simulation and commercial workloads show that speculation is a viable approach for simplifying system design. Systems can incur as many as ten recoveries per second due to mis-speculations without significantly degrading performance, and our speculatively simplified designs incur far fewer recoveries.



Date Posted: 22 May 2005