CSE502_lec19-directory+syncS10

CSE502_lec19-directory+syncS10 - CSE 502 Graduate Computer...

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Unformatted text preview: CSE 502 Graduate Computer Architecture Lec 19 – Directory-Based Shared-Memory Multiprocessors & MP Synchronization Larry Wittie Computer Science, StonyBrook University http://www.cs.sunysb.edu/~cse502 and ~lw Slides adapted from David Patterson, UC-Berkeley cs252-s06 4/14/10 CSE502-S10, Lec 19-Directory MP+Sync 2 Review & Assignment • Caches contain all information on state of cached memory blocks • Snooping cache over shared medium for smaller MP by invalidating other cached copies on write • Sharing cached data ⇒ Coherence (values returned by a read), Consistency (when a written value will be returned by a read) • Reading Assignment: Finish Chap. 4 MPs and start Chap 5 Memory Hierarchies. 4/14/10 CSE502-S10, Lec 19-Directory MP+Sync 3 Outline • Review • Directory-based protocols and examples • Synchronization • Relaxed Consistency Models • Fallacies and Pitfalls • Cautionary Tale • Conclusion 4/14/10 CSE502-S10, Lec 19-Directory MP+Sync 4 A Cache Coherent System Must: • Provide set of states, state transition diagram, and actions • Manage coherence protocol – (0) Determine when to invoke coherence protocol – (a) Find info about state of block in other caches to determine action » whether need to communicate with other cached copies – (b) Locate the other copies – (c) Communicate with those copies (invalidate/update) • (0) is done the same way on all systems – state of the line is maintained in the cache – protocol is invoked if an “access fault” occurs on the line • Different approaches distinguished by (a) to (c) 4/14/10 CSE502-S10, Lec 19-Directory MP+Sync 5 Bus-based Coherence • All of (a), (b), (c) done through broadcast on bus – faulting processor sends out a “search” – others respond to the search probe and take necessary action • Could do it in scalable network too – broadcast to all processors, and let them respond • Conceptually simple, but broadcast does not scale with p – on bus, bus bandwidth does not scale – on scalable network, every fault leads to at least p network transactions • Scalable coherence: – can have same cache states and state transition diagram – different mechanisms to manage protocol 4/14/10 CSE502-S10, Lec 19-Directory MP+Sync 6 Scalable Approach: Directories • Every memory block has associated directory information – keeps track of copies of cached blocks and their states – on a miss, find directory entry, look it up, and communicate only with the nodes that have copies if necessary – in scalable networks, communication with directory and copies is through network transactions • Many alternatives for organizing directory information 4/14/10 CSE502-S10, Lec 19-Directory MP+Sync 7 Basic Operation of Directory • k processors (or k snoopy nodes)....
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This note was uploaded on 11/06/2010 for the course CSE 502 taught by Professor Wittie,l during the Spring '08 term at SUNY Stony Brook.

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CSE502_lec19-directory+syncS10 - CSE 502 Graduate Computer...

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