This document discusses wormhole routing in multicomputers. In wormhole routing, packets are divided into fixed-size units called flits. The header flit determines the route and reserves channels as subsequent flits follow in a pipeline. Each channel can buffer only a few flits. Wormhole routing has lower latency than store-and-forward but can cause deadlock. Deadlock is addressed through preemption and rerouting of blocked packets or using deadlock-free routing algorithms like the turn model. Virtual channels allow sharing of physical channels among packets through time-division multiplexing of flit buffers.

1. • Each packet in a message is divided into flits (flow control digits)
• Size of a flit is at least log2 N in an N-processor multicomputer.
• Header flit governs the route, remaining flits follow in a pipeline fashion
• Channels in the path are reserved till the last flit in the packet traverses the channel.
• Each channel has enough storage to buffer only a few flits.
Advantages:
Lower Latency
Small flit Buffer at nodes
Channel can be shared by many
packets by Virtual channels
Disadvantages:
Deadlock
Solution :
Allow preemption of packets involved
in Deadlock & Reroute/discard
preempted packets.
Use deadlock free routing
Wormhole Routing

2. Handshaking and Latency
Adjacent nodes communicate requests and
acknowledgements using a one-bit ready/request (R/A)
line.
• When a receiver is ready, the R/A line low.
• When the sender is ready, R/A line is high &
next flit transmitted; the line is left high.
• After the receiver is done with the flit, R/A line is
lowered to indicate its ready to accept another flit.
The cycle repeats for transmission of other flits.
Latency:
• Store and forward latency is L/W (D+1),
• Wormhole latency is L/W + F/W D.
where
D = No. of intermediate nodes b/w source & destination
L = packet length (in bits)
F = flit length (in bits)
W = the channel bandwidth (in bits/sec)
F<<D, hence D has no significant effect on latency in
wormhole systems

3. Virtual Channels
 The channels between nodes in a wormhole-routed multicomputer are shared by many possible
source and destination pairs.
 A “virtual channel” is a pair of flit buffers (in nodes) connected by a shared physical channel.
 The physical channel is “time shared” by all the virtual channels.
 Other resources (including the R/A line) must be replicated for each of the virtual channels.
Deadl0ck Solutions:
Use deadlock free routing eg. TURN Model
Basis:
Analyze directions in which packets can turn in the network
Determine the cycles that such turns can form
Prohibit just enough turns to break all cycle
Resulting routing algorithms are:
Deadlock and livelock free
Minimal/Non-minimal
Highly Adaptive.
Ref: The Turn Model for Adaptive Routing by Christopher J. Glass and Lionel M. Ni