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Multicast Snooping

E. Bilir, R. Dickson, Y. Hu, M. Plakal, D. Sorin, M. Hill, D. Wood Presented By Derek Hower. Multicast Snooping. Why Multicast?. Goal: Reduce communication overhead in cache coherent multiprocessors Scalable snooping Reduced latency directories Solution:

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Multicast Snooping

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  1. E. Bilir, R. Dickson, Y. Hu, M. Plakal, D. Sorin, M. Hill, D. Wood Presented By Derek Hower Multicast Snooping

  2. Why Multicast? • Goal: • Reduce communication overhead in cache coherent multiprocessors • Scalable snooping • Reduced latency directories • Solution: • Hybrid snoop/directory protocol

  3. What is it? • Replace snooping bus with Multicast Address Network • Predict snoop transaction participants • Backup speculation with directory • Back end is Point-to-point data network (like Starfire)

  4. The Protocol • Snooping communication only with processors thought to be involved in the transaction • assume transaction is correct until told otherwise • Incorrect predictions are handled via nack and semiack • Small, predictive, directory protocol backs up the speculative snooping

  5. Mask Prediction • Node locality makes prediction feasible • local data (stack, some parts of the heap) • misses to the same block • Sticky-Spatial(k) prediction • Tracks block access, last invaldator • Introduced locality by using adjacent blocks in the prediction table • Possible for unrelated block to influence prediction • Memory corrects mistakes

  6. Address Network • Built as a fat tree (Modified Isotach) • Total ordering accomplished with timestamps • no need for synchronized delivery • Capable of multiple broadcasts in parallel

  7. Evaluation • “Big picture” simulations • mean number of sharers • prediction capability • mask set size • network availability • Simulated a MSI (not MOSI) protocol • only hurts results

  8. Results • Prediction accuracy: 73 – 95% • Avg. Nodes in Multicast: 2.4 – 5.6 (out of 32) • Avg. excess nodes predicted: 0.3 – 3.4 • Implementation better than half of optimal

  9. Deep Thinking • Evaluation of specifics • Timing: what if time to traverse fat tree overwhelms the benefits of decreased communication? • Complexity • What is the range (in system size) for which the benefits of multicast networking overcome complexity • Much room for improvement: • Better prediction • Smarter address network

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