Symmetry and performance in consistency protocols

A consistency protocol can be termed symmetric if all processors are treated identically when they access common resources. By contrast, asymmetric protocols usually assign a home or manager to each resource. Use of the resource by the home incurs less overhead than use by other processors. The key to good performance in such systems is to ensure that the asymmetry of the underlying protocol is skewed in the same way as that of the application. This paper presents a comparative evaluation of a symmetric and an asymmetric DSM protocol. We pay particular attention to those performance differences caused by symmetric and asymmetric features of the protocols. We then present the design and evaluation of an improved asymmetric writer protocol that dynamically migrates ownership according to access patterns. We show that the new protocol outperforms and is more stable than the non-migrating asymmetric protocol, and has much less memory overhead than the symmetric protocol.