Limits to the performance of software shared memory: a layered approach

Much research has been done in fast communication on clusters and in protocols for supporting software shared memory across them. However, the end performance of applications that were written for the more proven hardware-coherent shared memory is still not very good on these systems. Three major layers of software (and hardware) stand between the end user and parallel performance, each with its own functionality and performance characteristics. They include the communication layer, the software protocol layer that supports the programming model, and the application layer. These layers provide a useful framework to identify the key remaining limitations and bottlenecks in software shared memory systems, as well as the areas where optimization efforts might yield the greatest performance improvements. This paper performs such an integrated study, using this layered framework, for two types of software distributed shared memory systems: page-based shared virtual memory (SVM) and fine-grained software systems (FG). For the two system layers (communication and protocol), we focus on the performance costs of basic operations in the layers rather than on their functionalities. This is possible because their functionalities are now fairly mature. The less mature applications layer is treated through application restructuring. We examine the layers individually and in combination, understanding their implications for the two types of protocols and exposing the synergies among layers.