CSE 291-a Interconnection Networks

1. CSE 291-aInterconnection Networks Lecture 10: Flow Control February 21, 2007 Prof. Chung-Kuan Cheng CSE Dept, UC San Diego Winter 2007 Transcribed by Thomas Weng

2. Topics • Introduction • Bufferless Flow Control • Buffered Flow Control - Cut-through - Wormhole - Virtual Channel

3. Introduction • Objective: Bandwidth and Latency • Resources: Buffer and Channel State Switch Channel Channel Buffer

4. Unit of Messages Message Message is too long, cut up message Packet Basic chunk is Packet RI SN Header Body (content) Tail (indicates packet is done) RI = Routing Information SN = Sequence Number Packet should be reasonably long so we minimize overhead.

5. Flit and Phit Packet RI SN Packet Tail Flit Type Virtual Channel Flit (Flow Control Digit) H, T, or H&T, or Body Phit Phit (Physical Transfer Digit) Flits Packet Phits Typical Range 8 bits 1-64 bits 64 bits 16-512 bits 1K 128 bits–512K

6. Bufferless Flow Control • What if we have no buffer? • Why not have buffer? Less power and improved latency. • Three methods: 1. Drop the data (easiest way) 2. Misroute the data (treat data like hot potato) 3. Dropless approach (reservations)

7. Drop the Data Drop the data: Tell sender you dropped data 1) Nack – Negative Acknowledgement (Tell sender you dropped the data) 2) Ack – Acknowledgement (The sender resends if Ack is not received within timeout period) 0 0 c b a o c b a 0 0 1 1 h 8 f 0 h Drop dropped data Reverse Channel: Ack and flow control signals are sent in a reverse channel

8. Drop the Data (cont) Suppose channel 2 sent and was rejected 0 1 2 3 Dropped data simplifies things, but pay a price by resending data. Latency is long if data was rejected.

9. Dropless Flow Control Dropless Flow Control: Request propagates from source to destination and allocates the channel. Ack is transmitted back to the source. Packet is sent. A tail flit is sent to de-allocate the channel. Channel Total time T0 = 3Htr + L/b bandwidth length of packet hops latency / hop

10. Buffered Flow Control 1. Store & Forward 2. Cut-through 3. Wormhole 4. Virtual Channel

11. Store & Forward 1. Store & Forward Flow Control: Each node receives a packet and then sends it out. Channel T0 = H(tr + L/b)

12. Cut-through 2. Cut-through Flow Control: Each node starts to send the packet without waiting for the whole packet to arrive. Cut-through is more efficient approach. 1) Good performance 2) Large buffer sizes, consumes more power Suppose in the middle, we get stuck T0 = Htr + L/b

13. Wormhole 3. Wormhole Flow Control: Main difference is that we just have a little buffer, don’t need to store the entire packet. In terms of performance & bandwidth, it’s better than cut-through. States: Idle, Wait, Active I L W U L 0 0 T B B H T B B H 1 1

14. Wormhole (cont) W U L A U U 0 0 T B T H B H B B 1 1 A U U 0 In: B B H H B B T T 1 Out: H B B T

15. Virtual Channel 4. Virtual Channel: Try to split channel in time domain. By doing so we can fully utilize channel since we don’t waste it by holding it. Wormhole + Virtual Channel = Winner

16. Virtual Channel (cont) 2 1 A B 3 4 Input a an a1 a2 a3 a4 Input b bn b1 b2 b3 b4 Interleaved an bn a1 b1 a2 b2 a3 b3 a4 b4 Winner Takes All an a1 a2 a3 a4 bn b1 b2 b3 b4