Technical Report CS0767

Authors: H. Attiya and R. Friedman

Hybrid Consistency, a consistency condition for shared memory multiprocessors, attempts to capture the guarantees provided by contemporary high-performance architectures. It combines the expressiveness of strong consistency conditions (e.g., sequential consistency, linearizability) and the efficiency of weak consistency conditions (e.g., Pipelined RAM, causal memory). Memory access operations are classified as wither strong or weak. A global ordering of strong operations at different processes is guaranteed, but there us very little guarantee on the ordering of weak operations at different processes, except for what is implied by their interleaving with the strong operations. A formal and precise definition of this condition is given and an efficient algorithm for providing hybrid consistency on distributed memory machines is presented. The response time of the algorithm is proven to be within a constant multiplicative factor of the optimal time bounds. To motivate hybrid consistency, it is shown that weakly consistent memories do not support non-cooperative (in particular, non-centralized) algorithms for mutual exclusion.

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