Computer Architecture: Lecture 22 - Interconnects

Overview

• Topic: Introduction to interconnects in computer systems.
• Importance: Interconnects are critical in determining the performance and scalability of computer systems.
• Relation to Memory: Interconnects play a significant role in memory systems as well.

Key Concepts

Multiprocessing Fundamentals Recap

• Memory Ordering: Ensures consistency in data access.
• Cache Coherence: Techniques discussed include Snoopy buses and directory-based systems.
  • Snoopy Bus: Caches monitor a shared bus to maintain coherence.
  • Scalability Issue: Effectiveness reduces as the number of cores increases.
  • Directory-Based: Involves consulting a directory for memory access.

Interconnects

• Types: Can be on-chip or within entire systems (e.g., supercomputers).
• Usage: Connect components like processors, memory banks, I/O devices, etc.
• Impact:
  • Scalability: Affects how well systems can expand.
  • Performance: Influences latency, throughput, and efficiency.
  • Reliability & Security: Potential for failures and data leaks.

Topologies and Routing

Topology Types

• Regular vs. Irregular: Regular networks have a consistent structure.
• Routing Distance: Measured as the number of hops.
• Diameter: Maximum routing distance.
• Bisection Bandwidth: Used to evaluate network performance; measures the minimal cut required to divide the network.

Network Types

• Bus: Simple but not scalable for larger systems.
• Point-to-Point: Direct links between nodes, highly expensive.
• Crossbar: Combines features of both, used in small systems.
• Multistage Networks: Cost-effective with multiple stages, e.g., Omega, Butterfly.

Routing Techniques

• Deterministic Routing: Fixed path for source-destination pairs.
• Adaptive Routing: Adjusts based on network state; can be minimal (only shortest paths) or non-minimal.
• Avoiding Deadlock: Important in routing algorithm design.
  • Deadlock Avoidance: Use routing algorithms that prevent circular dependencies.

Flow Control

• Circuit Switching: Sets up a path before transmission.
• Packet Switching: Routes packets dynamically without pre-established paths.
• Bufferless Networks: Handle packets without buffer storage, using deflection.

Performance Metrics

• Latency: Considers routing distances and contention.
• Throughput: Maximum rate of successful message delivery.
• Saturation Point: Where increased load leads to higher latency.

Administrative Notes

• Exam Details:
  • Date: December 19
  • Time: 1:00 PM to 4:00 PM
  • Location: ML F39 (backup room ML F38)
• Grading Schedule: Final grades expected by early February; solutions to be released for homework.

Recommended Readings

• Papers on routing techniques and interconnection network designs.
• Suggested textbooks for deeper understanding of interconnections.

Conclusion

• Emphasized importance of interconnects in system performance.
• Next session will focus on on-chip networks and specific design challenges.