Multicast Snooping

1
Multicast Snooping

• A New Coherence Method Using A Multicast Address
  Network

2
Outline

• Multicast Snooping View from Above
• Multicast Snooping Details
• Experimental Implementation and Methodology
• Questions Posed by Multicast Snooping

3
View from Above

• Multicast Snooping
• A Hybrid of Snooping and Broadcast
• Performance benefits of snooping
• Scalability of broadcast
• Graceful degradation to broadcast as system grows
• Multicast Groups ala Networks
• Processor guesses peer(s) that need to see
  message, then multicasts to these targets
• Memory has directory to evaluate guesses, acts
  upon those that are wrong

4
View from Above, Continued

• Advantages of Multicast Snooping
• When Multicast Guess is Right, Speed of
  Snooping Protocol Achieved with Greater Scaling
  Over Traditional Snooping Systems
• Guessing right isnt that hard
• Net result is support of larger systems with
  snooping (better snooping scalability)
• When Multicast Guess is Wrong, Directory-Based
  Mechanisms Maintain Correctness
• Degradation as system scales to directory-like
  behavior

5
Outline

• Multicast Snooping View from Above
• Multicast Snooping Details
• Experimental Implementation and Methodology
• Questions Posed by Multicast Snooping

6
Multicast Snooping Coherence

• Logically Separate Multicast Address and Data
  Busses
• Authors Model Physical Separation for Simplicity
• MOSI Protocol
• Why MOSI? Why not MSI or MESI or MOESI?

7
Multicast Snooping Protocol

• Broadcast-Like, Three Major Differences
• Coherence Transactions Carry Mask
• Mask specification of which processors should
  receive the transaction always includes source
  processor and memory that owns the requested
  block
• Memory Carries Simplified Directory Entry
• Verifies mask is correct, reacts appropriately
• If incorrect, sends correct mask back with
  semi-ack or nack
• Processor Actions Carry Additional Complexity
• Needed to support semi-acks/nacks on getx

8
Multicast Snooping Mask Prediction

• Each Processor Maintains Local Mini-Directory
• Tracks Locality of Block Access, Last
  Invalidator, Arranged by Block Tag
• Builds Mask Using StickySpatial(k) Predictor
• Ors mask for block with masks for k-nearest
  neighbors in table
• Nearest neighbors may not be related blocks

9
Multicast Address Networks

• For Now, Consider As A Cloud
• Notable Utilization of Fat-Tree Network (Recall
  CM-5)
• Important Properties For Supporting Multicast
  Networks Include
• Total ordering, need not be simultaneous to all
  destinations
• Capable of multiple deliveries per cycle
• Low latency avoid bottlenecks, exploit locality

10
Outline

• Multicast Snooping View from Above
• Multicast Snooping Details
• Experimental Implementation and Methodology
• Questions Posed by Multicast Snooping

11
Performance Evaluation

• Not the Focus of the Paper
• Preliminary Evidence
• Suggests that Further Detailed Evaluation of
  Multicast Snooping is Warranted
• Simulated 32-Processor CC-NUMA
• Used Wisconsin WWT II Simulator
• MSI only pessimistic for Multicast Snooping
• Benchmarks Mainly Derived from SPLASH-2

12
Evaluating The Pieces

• Generated Traces Fed Through Mask Predictor
• Prediction Accuracy Range 73-95
• Extra Nodes Predicted Still Leave Multicast Group
  Size Much Smaller than System Size
• Would these results scale with smaller system
  sizes? Whats affecting the number of extra
  nodes?
• Network Results Show Multiple Messages Per Cycle
  Possible (50 of Optimal)

13
Outline

• Multicast Snooping View from Above
• Multicast Snooping Details
• Experimental Implementation and Methodology
• Questions Posed by Multicast Snooping

14
Questions To Consider

• Is Multicast Snooping A Good Idea?
• If So, Would It Be Better In Some Scenarios Than
  In Others?
• If Not, Why Not?
• Is Multicast Snooping Optimal?
• Are the Noted Drawbacks Regarding the Evaluation
  Back-Breakers?