Next: Introduction
Lazy Release Consistency for Hardware-Coherent Multiprocessors
Leonidas I. Kontothanassis,
Michael L. Scott,
and Ricardo Bianchini
Department of Computer Science
University of Rochester
Rochester, NY 14627-0226
{kthanasi,scott,ricardo}@cs.rochester.edu
December 1994
Abstract:
Release consistency is a widely accepted memory model for
distributed shared memory systems. Eager release consistency
represents the state of the art in release consistent protocols for
hardware-coherent multiprocessors, while lazy release
consistency has been shown to provide better performance for
software distributed shared memory (DSM). Several of the
optimizations performed by lazy protocols have the potential to
improve the performance of hardware-coherent multiprocessors as
well, but their complexity has precluded a hardware implementation.
With the advent of programmable protocol processors it may become
possible to use them after all. We present and evaluate a lazy
release-consistent protocol suitable for machines with dedicated
protocol processors. This protocol admits multiple concurrent
writers, sends write notices concurrently with computation, and
delays invalidations until acquire operations. We also
consider a lazier protocol that delays sending write notices until
release operations. Our results indicate that the first
protocol outperforms eager release consistency by as much as 20%
across a variety of applications. The lazier protocol, on the other
hand, is unable to recoup its high synchronization overhead. This
represents a qualitative shift from the DSM world, where lazier
protocols always yield performance improvements. Based on our
results, we conclude that machines with flexible hardware support
for coherence should use protocols based on lazy release
consistency, but in a less ``aggressively lazy'' form than is
appropriate for DSM.
Leonidas Kontothanassis
Mon Jul 24 22:40:09 EDT 1995