![]() ![]() In a 1996 paper, he introduced the design of the SGI Origin 2000, a family of server computers employing directory based cache coherence. In 1992, Daniel Lenoski from Stanford university published a paper proposing advances in cache coherence protocols for directory-based systems. In 1985, James Archibald and Jean-Loup Baer from the University of Washington published a paper that proposes a more economical, expandable, and modular variation of the "global directory" approach in the term of hardware use in the design. The limitations of other competitors made it easier for DASH based systems to get chosen when designing cache coherence systems and other systems needing scalability in cache-based nodes. Similarly, IBM PR3 does not provide hardware cache coherence, which limits the performance of both of these designs, especially when employing high performance processors. However, both of these systems have a drawback For example, BBN Butterfly does not have caches. ![]() For instance, BBN Butterfly which was introduced in 1985, and IBM PR3 which was introduced in 1987, are some examples of scalable multiprocessor systems. Beside this approach, several attempts were done to provide a scalable systems. However, applying it to cache coherence was proposed a few years later, specifically in 1978, when researchers at Stanford University proposed the first version of this coherence systems called Stanford DASH, in a paper that described the system with the difficulties and improvements associated with such designs. The idea of DASH ( Directory Architecture for SHared-memory) was first proposed by C.K. The idea of Directory-based cache coherence systems began long ago. So there is a kind of trade-off between the simplicity and the scalability when comparing between Bus-based and Directory-based cache coherence designs. However, directory based systems become crucial when the system scale up and the number of nodes grows. Since the bus structure itself can serve as an organizer for all the traffic that goes through the system, and ensure the atomicity of all the signals passed through, there will be no need to put more effort in ensuring atomicity and ordering between signals as the case in directory based systems, which leads to several overhead faced in the later system design when dealing with issues like consistency.Īccording to the above discussion, it is clear that using bus based systems seems more attractive for relatively small systems. Simplicity: This is one of the points where the bus-system is superior.On the other hand, using directory-based systems, there will be no such bottleneck to constrain the scalability of the system. Especially since only one node is allowed to use the bus at a time, which will significantly harm the performance of the overall system. However, while the number of nodes is growing, some problems may occur in this regard. For a relatively small number of nodes, bus systems can do well. For this criteria, Bus based systems cannot do well due to the limitation caused when having a shared bus that all nodes are using in the same time. What we mean by scalability, in short, is how good a specific system is in handling the growing amount of work that it is responsible to do. Scalability: This is one of the strongest motivations for going to directory based designs.In directory based cache coherence, this is done by using this directory to keep track of the status of all cache blocks, the status of each block includes in which cache coherence " state" that block is, and which nodes are sharing that block at that time, which can be used to eliminate the need to broadcast all the signals to all nodes, and only send it to the nodes that are interested in this single block.įollowing are a few advantages and disadvantages of the directory based cache coherence protocol: directory or bus) as a tool to facilitate the communication between different nodes, and to guarantee that the coherence protocol is working properly along all the communicating nodes. Both of these designs use the corresponding medium (i.e. Directory-based coherence uses a special directory to serve instead of the shared bus in the bus-based coherence protocols. Another popular way is to use a special type of computer bus between all the nodes as a "shared bus" (a.k.a. ( October 2018)ĭirectory-based coherence is a mechanism to handle Cache coherence problem in Distributed shared memory (DSM) a.k.a. Please introduce links to this page from related articles try the Find link tool for suggestions. This article is an orphan, as no other articles link to it. ![]()
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