A performance study of memory consistency models

Recent advances in technology are such that the speed of processors is increasing faster than memory latency is decreasing. Therefore the relative cost of a cache miss is becoming more important. However, the full cost of a cache miss need not be paid every time in a multiprocessor. The frequency with which the processor must stall on a cache miss can be reduced by using a relaxed model of memory consistency.

In this paper, we present the results of instruction-level simulation studies on the relative performance benefits of using different models of memory consistency. Our vehicle of study is a shared-memory multiprocessor with processors and associated write-back caches connected to global memory modules via an Omega network. The benefits of the relaxed models, and their increasing hardware complexity, are assessed with varying cache size, line size, and number of processors. We find that substantial benefits can be accrued by using relaxed models but the magnitudes of the benefits depend on the architecture being modeled, the benchmarks, and how the code is scheduled. We did not find any major difference in levels of improvement among the various relaxed models.