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Mechanisms of Flow Control in Virtual Channels

Explore different flow control strategies like virtual channels and deadlocks in information transmission networks. Learn how to avoid and manage deadlock situations efficiently.

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Mechanisms of Flow Control in Virtual Channels

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  1. EE382CLecture 8 Flow Control Part 2 Virtual Channels Mechanics of Flow Control Deadlock 4/21/11 EE 382C - S11- Lecture 8

  2. Logistics • HW2 due today • HW3 out today - see next slide - due 4/28 • Midterm IN CLASS on 5/3 EE 382C - S11- Lecture 8

  3. Homework 3 • Problems 13.3, 14.2, 14.9 EE 382C - S11- Lecture 8

  4. Pending Question from Tuesday • If storage is plentiful and channel bandwidth is expensive, what flow-control strategy should you use? • Hint: Does this depend on packet length? EE 382C - S11 - Lecture 7

  5. Question of the day • What is the minimum number of virtual channels needed to break deadlock in a mesh network using virtual-channel flow control? EE 382C - S11- Lecture 8

  6. Circuit Switching (II) EE 382C - S11 - Lecture 7

  7. Flow Control Review • Allocate resources • Channel bandwidth, buffer space, router state • To units of information • Packets or Flits • Unbuffered flow control • Circuit switching • Dropping • Misrouting • Buffers decouple channel allocation • Packet-Buffer Flow Control • Store-&-Forward • Virtual cut-through • Flit-Buffer Flow Control • Wormhole • Virtual Channel • Virtual channels decouple channel dependencies EE 382C - S11- Lecture 8

  8. Buffered Flow Control • Packet-Buffer FC • Store and Forward • Virtual Cut Through • Flit-Buffer FC • Wormhole • Virtual Channels EE 382C - S11 - Lecture 7

  9. Packet-Buffer Flow Control EE 382C - S11 - Lecture 7

  10. Store-and-Forward EE 382C - S11 - Lecture 7

  11. Virtual Cut-through EE 382C - S11 - Lecture 7

  12. Packet-Buffer Flow Control EE 382C - S11 - Lecture 7

  13. Allocation units EE 382C - S11 - Lecture 7

  14. What is good about packet buffers?What is bad about packet buffers? EE 382C - S11 - Lecture 7

  15. Wormhole The Movie EE 382C - S11 - Lecture 7

  16. Head flit arrives, allocate VC state, request upper output EE 382C - S11 - Lecture 7

  17. Body flit arrives, still waiting on upper output EE 382C - S11 - Lecture 7

  18. Output allocated, forward head flit, second body flit arrives EE 382C - S11 - Lecture 7

  19. Tail arrives, body flit forwarded EE 382C - S11 - Lecture 7

  20. Second body flit forwarded EE 382C - S11 - Lecture 7

  21. Tail flit forwarded, input and output VC states deallocated EE 382C - S11 - Lecture 7

  22. Virtual Channel Flow Control EE 382C - S11- Lecture 8

  23. Blocking of wormhole flow control EE 382C - S11 - Lecture 7

  24. Virtual-channel flow control decouples dependency between buffer and channel EE 382C - S11 - Lecture 7

  25. Sometimes, its better to be unfair (part 1) EE 382C - S07- Lecture 8

  26. Fair arbitration interleaves flits. Both packets are impeded EE 382C - S07- Lecture 8

  27. Winner-take-all arbitration impedes only one packet – by the same amount as interleaving EE 382C - S07- Lecture 8

  28. Q • When should you use winner-take-all? • And when shouldn’t you? EE 382C - S07- Lecture 8

  29. Is “Winner Take All” just Wormhole? EE 382C - S07- Lecture 8

  30. State of a VC router EE 382C - S07- Lecture 8

  31. Wide vs long buffers(Which of these is better?) EE 382C - S11- Lecture 8

  32. Many small buffers winsnon-interference with stiff backpressure EE 382C - S11- Lecture 8

  33. VCs as a Swiss Army knife EE 382C - S11- Lecture 8

  34. Pending Question from Tuesday • If storage is plentiful and channel bandwidth is expensive, what flow-control strategy should you use? • Hint: Does this depend on packet length? EE 382C - S11 - Lecture 7

  35. Mechanics of Flow Control EE 382C - S11- Lecture 8

  36. Credit-Based Flow Control EE 382C - S11- Lecture 8

  37. On/Off Flow Control EE 382C - S11- Lecture 8

  38. Ack-Nack Flow Control EE 382C - S11- Lecture 8

  39. Deadlock EE 382C - S11- Lecture 8

  40. Definition Deadlock • A condition in which a set of agents waits indefinitely trying to acquire a set of resources EE 382C - S11- Lecture 8

  41. Examples • Dining Philosophers • Circuit Switching • Who are the agents? • What are the resources? EE 382C - S11- Lecture 8

  42. Deadlocked Configuration • Waits-for, held-by graph • Circuit switching example EE 382C - S11- Lecture 8

  43. Example • Circuit-switched ring network EE 382C - S11- Lecture 8

  44. Resource Dependence A resource A is dependent on a resource B if it is possible for A to be held-by an agent X and it is also possible for X to wait-for B EE 382C - S11- Lecture 8

  45. Cyclic Resource Dependence A cycle in the resource dependence graph means that a deadlocked configuration is possible. It does not mean that deadlock has occurred or will occur. EE 382C - S11- Lecture 8

  46. Example • Consider the same 4-node ring with packet-buffer flow control • What is the dependence graph? • What is the held-by, wait-for graph for a deadlocked configuration? • How can you avoid deadlock in this network? EE 382C - S11- Lecture 8

  47. Example • Consider the same 4-node ring network with virtual channel flow control • What is the dependence graph? • How can you avoid deadlock in this network? EE 382C - S11- Lecture 8

  48. Protocol Deadlock EE 382C - S11- Lecture 8

  49. Next Time • Dealing with Deadlock • Two main approaches • Just say NO! (to deadlock) • AKA Deadlock Avoidance • Prevent a deadlock from ever occurring • e.g., by resource ordering • Fix it • AKA Deadlock Recovery • Detect a deadlock and correct it • e.g., by dropping packets or using “escape paths” EE 382C - S11- Lecture 8

  50. Question of the day • What is the minimum number of virtual channels needed to break deadlock in a mesh network using virtual-channel flow control? EE 382C - S11- Lecture 8

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