Abstract: "This paper considers alternative directory protocols for providing cache coherence in shared-memory multiprocessors with 32 to 128 processors, where the state requirements of Dir[subscript N] may be considered too large. We consider Dir[subscript i]B, i = 1,2,4, Dir[subscript N], Tristate (also called superset), Coarse Vector, and three new protocols. The new protocols -- Gray-hardware, Gray-software, Home -- are optimizations of Tristate that use gray coding to favor near-neighbor sharing. Our results are the first to compare all these protocols with complete applications (and the first evaluation of Tristate with a non- synthetic workload). Results for three applications -- ocean (one dimensional sharing), appbt (three-dimensional sharing), and barnes (dynamic sharing) -- for 128 processors on the Wisconsin Wind Tunnel show that (a) Dir1B sends 15 to 43 times as many invalidation messages as Dir[subscript N], (b) Gray-software sends 1.0 to 4.7 times as many messages as Dir[subscript N], making it better than Tristate, Gray- Hardware, and Home, and (c) the choice between Dir[subscript i]B, Coarse Vector, and Gray-software depends on whether one wants to optimize for few sharers (Dir[subscript i]B), many sharers (Coarse Vector), or hedge one's bets between both alternatives (Gray-software)."

Some alternative directory entry formats are described, including a special entry format for implementing queueing semaphores. Evaluation of the various entry formats is done with probabilistic models of shared cache blocks and software simulation. A variable length global table organization is presented which can be used to reduce the size of the global table, regardless of the entry format. Its performance is analyzed using software simulation. A protocol which maintains a linked list of processors which have a particular block cached is presented. Several variations of this protocol induce less interconnection network traffic than traditional protocols."

Euro-Par 2005 was the eleventh conference in the Euro-Par series. It was organized by the Centre for Informatics and Information Technology (CITI) and the Department of Informatics of the Faculty of Science and Technology of Universidade Nova de Lisboa, at the Campus of Monte de Caparica.

Directory-based cache consistency protocols have the potential to allow shared-memory multiprocessors to scale to a large number of processors. While many variations of these coherence schemes exist in the literature, they have typically been described at a rather high level, making adequate evaluation difficult. This paper explores the implementation issues of directory-based coherency strategies by developing a design at the level of detail needed to write a memory system functional simulator with an accurate timing model. The paper presents the design of both an invalidation coherency protocol and the associated directory/memory hardware. Support is added to prevent deadlock, handle subtle consistency situations, and implement a proper programming model of multiprocess execution. Extensions are delineated for realizing a multiple-threaded directory that can continue to process commands while waiting for a reply from a cache. The final hardware design is evaluated in the context of the number of parts required for implementation.

The book illustrates state-of-the-art software solutions for cache coherence maintenance in shared-memory multiprocessors. It begins with a brief overview of the cache coherence problem and introduces software solutions to the problem. The text defines and details static and dynamic software schemes, techniques for modeling performance evaluation mechanisms, and performance evaluation studies.