Table of Contents
Protocol Design Tradeoffs in Snooping Cache Coherent Multiprocessors
Recap
Sequential Consistency
Outline for Today
Write-back Caches
MSI Invalidate Protocol
Example: Write-Back Protocol
Correctness
Write Serialization for Coherence
Sequential Consistency
Sufficient conditions
Lower-level Protocol Choices
MESI (4-state) Invalidation Protocol
Hardware Support for MESI
MESI State Transition Diagram
Lower-level Protocol Choices
Update Protocols
Dragon Write-back Update Protocol
Dragon State Transition Diagram
Lower-level Protocol Choices
Assessing Protocol Tradeoffs
Workload-Driven Evaluation
Evaluation in Uniprocessors
More Difficult for Multiprocessors
A Lot Depends on Sizes
Scaling: Why Worry?
Too Large a Problem
Demonstrating Scaling Problems
Communication and Replication
Working Set Perspective
Working Set Curves (for given P)
Working Sets Change with P
Where the Time Goes: NPB LU-a
False Sharing Misses: Artifactual Comm.
Questions in Scaling
Under What Constraints to Scale?
Problem Constrained Scaling
Time Constrained Scaling
Memory Constrained Scaling
Types of Workloads
Coverage: Stressing Features
Coverage: Levels of Optimization
Concurrency
Workload/Benchmark Suites
Evaluating a Fixed-size Machine
Steps in Choosing Problem Sizes
Steps in Choosing Problem Sizes
Choosing Problem Sizes (contd.)
Evaluating an Idea or Tradeoff
Multiprocessor Simulation
Execution-driven Simulation
Difficulties in Simulation-based Evaluation
Choosing Parameters
Returning to protocol tradeoffs
Bandwidth per transition
Bandwidth Trade-off
Smaller (64KB) Caches
Cache Block Size
Miss Classification
Breakdown of Miss Rates with Block Size
Breakdown (cont)
Breakdown with 64KB Caches
Traffic
Traffic with 64 KB caches
Traffic SimOS 1 MB
Making Large Blocks More Effective
Update versus Invalidate
Update vs Invalidate: Miss Rates
Upgrade and Update Rates (Traffic)
Summary
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Author: David E. Culler
Home Page: http://www.cs.berkeley.edu/~culler/cs258-s99/
Other information:
David E. Culler UC Berkeley
CS258 Parallel Computer Architecture
Lecture 5
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