Boosting the Performance of Shared Memory Multiprocessors

Shared memory multiprocessors make it practical to convert sequential programs to parallel ones in a variety of applications. An emerging class of shared memory multiprocessors are nonuniform memory access machines with private caches and a cache coherence protocol. Proposed hardware optimizations to CC-NUMA machines can shorten the time processors lose because of cache misses and invalidations. The authors look at cost-performance trade-offs for each of four proposed optimizations: release consistency, adaptive sequential prefetching, migratory sharing detection, and hybrid update/invalidate with a write cache. The four optimizations differ with respect to which application features they attack, what hardware resources they require, and what constraints they impose on the application software. The authors measured the degree of performance improvement using the four optimizations in isolation and in combination, looking at the trade-offs in hardware and programming complexities. Although one combination of the proposed optimizations (prefetching and migratory sharing detection) can boost a sequentially consistent machine to perform as well as a machine with release consistency, release consistency models offer significant performance improvements across a broad application domain at little extra complexity in the machine design. Moreover, a combination of sequential prefetching and hybrid update/invalidate with a write cache cuts the execution time of a sequentially consistent machine by half with fairly modest changes to the second-level cache and the cache protocol. The authors expect that designers will begin to turn more to the release consistency model