The DASH Prototype: Implementation and Performance

The fundamental premise behind the DASH project is that it is feasible to build large-scale shared-memory multiprocessors with hardware cache coherence. While paper studies and software simulators are useful for understanding many high-level design trade-offs, prototypes are essential to ensure that no critical details are overlooked. A prototype provides convincing evidence of the feasibility of the design allows one to accurately estimate both the hardware and the complexity cost of various features, and provides a platform for studying real workloads. A 16-processor prototype of the DASH multiprocessor has been operational for the last six months. In this paper, the hardware overhead of directory-based cache coherence in the prototype is examined. We also discuss the performance of the system, and the speedups obtained by parallel applications running on the prototype. Using a sophisticated hardware performance monitor, we characterize the effectiveness of coherent caches and the relationship between an application's reference behavior and its speedup.