Can shared-memory model serve as a bridging model for parallel computation?

Abstract

There has been a great deal of interest recently in the development of general-purpose bridging models for parallel computation. Models such as the BSP and LogP have been proposed as more realistic alternatives to the widely-used PRAM model. The BSP and LogP models imply a rather different style for designing algorithms when compared to the PRAM model. Indeed, while many consider data parallelism as a convenient style, and the shared-memory abstraction as an easy-to-use platform, the bandwidth limitations of current machines have diverted much attention to message-passing and distributed-memory models (such as the BSP and LogP) that account more properly for these limitations. In this paper we consider the question of whether a shared-memory model can serve as an effective bridging model for parallel computation. In particular, can a shared-memory model be as effective as, say, the BSP? As a candidate for a bridging model, we introduce the Queuing Shared Memory (QSM) model, which accounts for limited communication bandwidth while still providing a simple shared-memory abstraction. We substantiate the ability of the QSM to serve as a bridging model by providing a simple work-preserving emulation of the QSM on both the BSP, and on a related model, the (d,x)-BSP. We present evidence that the features of the QSM are essential to its effectiveness as a bridging model. In addition, we describe scenarios in which the high-level QSM more accurately models certain machines than the more detailed BSP and LogP models. Finally, we present algorithmic results for the QSM, as well as general strategies for mapping algorithms designed for the BSP or PRAM models onto the QSM model. Our main conclusion is that shared-memory models can potentially serve as viable alternatives to existing message-passing, distributed-memory bridging models.