Low-Cost Support for Fine-Grain Synchronization in Multiprocessors

As multiprocessors scale beyond the limits of a few tens of processors, they must look beyond traditional methods of synchronization to minimize serialization and achieve the high degrees of parallelism required to utilize large machines. By allowing synchronization at the level of the smallest unit of memory, ne-grain synchronization achieves these goals. Unfortunately, supporting e cient ne-grain synchronization without inordinate amounts of hardware has remained a challenge. This paper describes the support for ne-grain synchronization provided by the Alewife system. The premise underlying Alewife's implementation is that successful synchronization attempts are the common case when serialization is minimized through word-level synchronization. For our applications, the failure rates were less than 7%. E ciency at low hardware cost is achieved by providing hardware support to streamline successful synchronization attempts and relegating other non-critical operations to software. Alewife provides a large synchronization name space by associating full/empty bits with each memory word. Successful synchronization attempts execute at normal load-store speeds, while attempts that fail invoke appropriate software trap handlers through a fast trap mechanism. The software handlers deal with the issues of retrying versus blocking, queueing, and rescheduling. The e ciency of Alewife's mechanisms is analyzed by comparing the costs of various synchronization operations and parallel application execution time. In several applications we studied, our hardware support improved performance by 35%{50%.