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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