A correctness condition for high-performance multiprocessors (extended abstract)

Hybrid consistency, a new 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 either strong or weak. A global ordering of strong operations at different processes is guaranteed, but there is 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. An efficient implementation of hybrid consistency on distributed memory machines is presented. In this implementation, weak opearations are executed instantaneously, while the response time for strong operations is linear in the network delay. (It is proven that this is within a constant 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.