Evaluating the Benefits of Communication Coprocessors

Abstract

Communication coprocessors (CCPs) have become commonplace in modern MPPs and networks of workstations. These coprocessors provide dedicated hardware support for fast communication. In this paper we study how to exploit the capabilities of CCPs for executing user-level message handlers. We show, in the context of Active Messages and Split-C, that we can move message-handling code to the coprocessor, thus freeing the main processor for computational work. We address the important issues that arise, such as synchronization, and the limited computational power and flexibility of CCPs. We have implemented coprocessor versions of both Active Messages and Split-C. These implementations, developed on the Meiko CS-2, provide us with an excellent experimental platform to evaluate the benefits of a communication coprocessor architecture. Our experimental results show that the coprocessor version of Split-C is more responsive than the version which executes all message handlers on the main processor. This is despite the fact that some communication primitives are slower since the coprocessor on the Meiko CS-2 computes much slower. Overall, the performance on large Split-C applications is better because the coprocessor reacts faster and offloads the main processor, which does not need to poll or process interrupts.