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

2. Why Multicast? Goal: Solution:
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 – (out of 32)
Avg. excess nodes predicted: 0.3 – 3.4
Implementation better than half of optimal

9. Deep Thinking Evaluation of specifics Complexity
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