Software-extended coherent shared memory: performance and cost

Evaluates the tradeoffs involved in the design of the software-extended memory system of Alewife, a multiprocessor architecture that implements coherent shared memory through a combination of hardware and software mechanisms. For each block of memory, Alewife implements between zero and five coherence directory pointers in hardware and allows software to handle requests when the pointers are exhausted. The software includes a flexible coherence interface that facilitates protocol software implementation. This interface is indispensable for conducting experiments and has proven important for implementing enhancements to the basic system. Simulations of a number of applications running on a complete system (with up to 256 processors) demonstrate that the hybrid architecture with five pointers achieves between 71% and 100% of full-map directory performance at a constant cost per processing element. The authors experience in designing the software protocol interfaces and experiments with a variety of system configurations lead to a detailed understanding of the interaction of the hardware and software components of the system. The results show that a small amount of shared memory hardware provides adequate performance: One-pointer systems reach between 42% and 100% of full-map performance on parallel benchmarks. A software only directory architecture with no hardware pointers has lower performance but minimal cost