Bandwidth doesnt scale on scalable network every

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Unformatted text preview: in scalable network too do it in scalable network – broadcast to all processors, and let them respond • Conceptually simple, but broadcast doesn’t scale with p – on bus, bus bandwidth doesn’t scale – on scalable network, every fault leads to at least p network transactions • Scalable coherence: – can have same cache states and state transition diagram sa cac states state – different mechanisms to manage protocol 18 Scalable Approach: Directories • Every memory block has associated directory information 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 transactions • Many alternatives for organizing directory information Assume physical-address caches 19 Basic Operation of Directory P P Cache Cache • k processors. processors • With each block in memory: k presence-bits, 1 dirty-bit Interconnection Network Memory •• presence bits • Directory • With each cache-block in a cache: With each cache in cache 1 valid bit, 1 dirty/owner bit dirty bit • Read from main memory by processor i: • If dirty-bit OFF then { read from main memory; turn p[i] ON; } • if dirty-bit ON then { recall line from dirty proc (cache state to shared); update memory; turn dirty-bit OFF;...
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