1、 Recognized as anAmerican National Standard (ANSI)IEEE Std 1596-1992(Adopted by ISO/IEC and redesignated asISO/IEC 13961:2000)IEEE Standard for Scalable CoherentInterface (SCI)SponsorMicroprocessor and Microcomputer Standards Subcommitteeof theIEEE Computer SocietyApproved 19 March 1992IEEE-SA Stand
2、ards BoardAdopted by ISO/IEC and redesignated asISO/IEC 13961:2000Abstract:The scalable coherent interface (SCI) provides computer-bus-like services but, insteadof a bus, uses a collection of fast point-to-point unidirectional links to provide the far higher through-put needed for high-performance m
3、ultiprocessor systems. SCI supports distributed, sharedmemory with optional cache coherence for tightly coupled systems, and message-passing forloosely coupled systems. Initial SCI links are defined at 1 Gbyte/s (16-bit parallel) and 1 Gb/s(serial). For applications requiring modular packaging, an i
4、nterchangeable module is specifiedalong with connector and power. The packets and protocols that implement transactions aredefined and their formal specification is provided in the form of computer programs. In addition tothe usual read-and-write transactions, SCI supports efficient multiprocessor l
5、ock transactions. Thedistributed cache-coherence protocols are efficient and can recover from an arbitrary number oftransmission failures. SCI protocols ensure forward progress despite multiprocessor conflicts (nodeadlocks or starvation).Keywords:bus architecture, bus standard, cache coherence, dist
6、ributed memory, fiber optic,interconnect,I/O system, link, mesh, multiprocessor, network, packet protocol, ring, seamlessdistributed computer,shared memory, switch, transaction setThe Institute of Electrical and Electronics Engineers, Inc.3 Park Avenue, New York, NY 10016-5997, USACopyright 2001 by
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25、troduction(This introduction is not a part of IEEE Std 1596-1992, IEEE Standard for Scalable Coherent Interface SCI.)The demand for more processing power continues to increase, and apparently has no limit. One can usefully saturatethe resources of any computer so easily by merely specifying a finer
26、mesh or higher resolution for the solution of somephysical problem (hydrodynamics, for example), that engineers and scientists are desperate for enormously largercomputers.To get this kind of computing power, it seems necessary to use a large number of processors cooperatively. Because ofthe propaga
27、tion delays introduced when signals cross chip boundaries, the fastest uniprocessor may be on one chipbefore long. Pipelining and similar large-mainframe tricks are already used extensively on single-chip processors.Vector processors help, but are hard to use efficiently in many applications. Multip
28、rocessors communicating bymessage passing work well for some applications, but not for all. The shared-memory multiprocessor looks like thebest strategy for the future, but a great deal of work will be needed to develop software to use it efficiently.It is important to support both the shared-memory
29、 and the message-passing models efficiently (and at the same time)in order to support optimal software for a wide range of problems, especially for a system that dynamically allocatesprocessors and perhaps changes its configuration depending on the nature of its load.SCI started from an attempt to i
30、ncrease the bandwidth of a backplane bus past the limits set by backplane physics inorder to meet the needs of new generations of processor chips, some of which can single-handedly saturate the fastestbuses. We soon learned that we had to abandon the bus structure to achieve our goals.Backplane perf
31、ormance is limited by physics (distributed capacitances and the speed of light) and by a buss one-at-a-time nature, an inherent bottleneck. To gain performance far beyond what buses and backplanes can do, one needsbetter signaling techniques and the concurrent use of many signaling paths.Rather than
32、 using bused backplane wires, SCI is based on point-to-point interconnect technology. This designapproach eliminates many of the physics problems and results in much higher speeds. SCI in effect simulates a bus,providing the bus services one expects (and more) without using buses.SCI has turned out
33、to be surprisingly simple, much simpler than many of the alternative designs we explored and muchsimpler than bus-based systems would be if they tried to approach a comparable size and performance. This simplicitymay not be obvious to the first-time reader of this rather thick document, but much of
34、this bulk is due to the largeamount of tutorial material necessary to introduce such a new way of doing things (a paradigm shift), and even moreis due to the comprehensive executable description of cache behavior under all possible conditions.The switch from a shared backplane bus to a point-to-poin
35、t interconnect has created many new problems and researchtopics, which have been resolved in record time by this SCI project. Much research remains to be done on determiningoptimal ways to use the mechanisms SCI provides. SCI has also required the development of novel allocation andcache-coherence p
36、rotocols, which has made the project a challenging one indeed, particularly in view of our scheduleobjectives.Historical Perspective and AcknowledgmentsMost of the developers of SCI come from high-speed-bus backgrounds, such as Fastbus (IEEE Std 960-1989) orFuturebus (IEEE Std 896.1-1987). Paul Swea
37、zey, who was the coordinator of the Futurebus cache coherence taskgroup, initiated a SuperBus Study Group under the IEEE Computer Societys Microprocessor Standards Committee inNovember 1987 to consider whether something could be done for the next bus generation to avoid the multitude ofcompeting inc
38、ompatible standards we saw in the 32-bit generation. Futurebus tried to solve that problem, starting inthe late 1970s, but could not converge to a single best solution in time to head off the development of manyalternatives.ivThe SuperBus Study Group met for less than a year before deciding that the
39、re was indeed a way to do better and toachieve the throughput rates that are required for supporting multiple 100-MFLOPS-class processor chips, namelyabout 1 Gbyte/s per processor. We were particularly urged on by Paul L. Borrill, Futurebus chairman, and JohnMoussouris (one of the founders of MIPS),
40、 who frightened us all by his predictions of immensely powerful processorsin the near futurewhich already are coming true!Our July 1988 Project Authorization Request was approved by the IEEE Standards Board in October. David B.Gustavson was appointed Chairman and David V. James became the logical-ta
41、sk-group coordinator and ViceChairman. Gustavson also served as physical-task-group coordinator, handled the records and mailings, and sharedminutes-taking and editing duties with David James.A Control and Status Register and I/O Architecture effort was started within SCI, based on some significantc
42、ontributions by David James. When it was recognized as important for other standard buses as well, it was split off asan independent activity shared by Futurebus+, Serial Bus (P1394), and others. In April 1989 this also became anofficial project, P1212, with David James as chairman. The goal of a un
43、iform CSR architecture has been attemptedmany times before (e.g., by the Fastbus Software Working Group, chaired by Gustavson), and has proven elusive. Thereason P1212 has had a more comprehensive success is that David James brought considerable architecturalexperience to bear, generating sufficient
44、 rationale for the various choices so that decisions no longer seem entirelyarbitrary. Much of this rationale is a consequence of multiprocessor architectural considerations; without the constraintof efficient multiprocessor interoperability, many CSR design issues would be too arbitrary to be able
45、to achieve timelystandardization.The CSR Architecture has become a unifying force for the latest generation of buses, encouraging VME andMULTIBUSII users to use the CSR architecture as they interface to Futurebus+, thus facilitating a future interface toSCI as system requirements grow. In this way,
46、there is a relatively smooth and well-defined growth path from present-generation single-processor systems through Futurebus+s several-processor systems with cache coherence, to SCIsmany-processor systems. Because of the importance of such a migration path to the future acceptance of SCI, we placehi
47、gh priority on interfacing SCI with other buses. For that reason we include protocol hooks that would not otherwisebe needed. In exchange, SCI users will be able to take advantage of the large number of existing I/O interfaces.In March 1989, a Fiber Optic Task Group (SCI-FI) was started, led by Hans
48、 Wiggers, and an SCI/Futurebus+ BridgeTask Group was started, led by Mark Williams (a joint appointment with Futurebus+).Throughout the development of SCI, Knut Alnes and Ernst Kristiansen were working on an early implementation,providing input for the details of the specification. They also initiat
49、ed work at the University of Oslo, by Stein Gjessingand others, on formal verification of the cache-coherence mechanisms. This real implementation effort was extremelyvaluable to SCI, and greatly accelerated convergence to a practical specification.David James generated documents at an incredible rate. As the result of his single-handed effort the bulk of the text ofthis specification first appeared in June 1989. At the same time he was producing two volumes of similar size for theCSR working group! He is convinced that having something on paper produces more productive di
