Network characterized by:

1

• Network characterized by
• Topology
• Physical structure of the graph
• Routing Algorithm
• What paths through network can a message follow
• Switching Strategy
• How data in message traverses its route
• (Circuit Switched vs Packet Switched)
• Flow Control
• When does a packet (or portions of it) move along
  its route

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Things you would want a routing algorithm to have

• Minimize the number of hops that are required for
  the packet to reach its destination (ideally,
  forces the packet to get closer to its
  destination on every hop)
• Deadlock and livelock prevention
• Route around network faults
• Balance the load on network resources

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Deterministic vs. Adaptive

• Deterministicfollows a pre-specified route
• K-ary n-cube dimension-order routing
• Adaptiveroute determined by dynamic conditions
  of the network

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Adaptive Routing Load Balancing
adaptive
deterministic

• Routing based on current network conditions gives
• a clear advantage in terms of load balancing

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Deadlock/Livelock prevention

• Livelock Packet can't progress towards its
  destination because it is forever denied of the
  resources it needs to progress
• Deadlock Packet is blocked by packets that are
  blocked. There is a cycle of blockings.

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Deadlock prevention and Wormhole routing

• Particularly hard with wormhole routing
• A packet could be blocking several links
• Cant assemble the packet for later transmission
  (buffers are too small)

7
Deadlock preventive schemes

• Deterministic
• Easy Make sure algorithm is acyclic.
• Hard Detect deadlock (timer). Backpressure flow
• Adaptive
• Restrict adaptivity. Still adaptive, but in a way
  that's proven to be acyclic.
• Maybe add virtual channels to expand adaptivity
  once all cycles have been eliminated

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Restricting Adaptivity

• Greatly reduce hardware requirement for deadlock
  prevention

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The Turn Algorithm

• The Turn Model for Adaptive Routing (1992)
• Christopher J. Glass, Lionel M. Ni
• A turn is defined as a specific as a specific
  pair of input output links around a node
• In order to break all the cycles in a network, it
  is sufficient to prohibit a subset of all
  possible turns
• Adaptivity is restricted, but deadlock freedom is
  achieved

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The Turn Algorithm2D example (mesh)
Giving 2 possible turns
Node can route in any of 4 directions (west,
east, north, south)
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The Turn Algorithm2D example (mesh)
One possibility
Prevents all cycles, but doesnt allow any
adaptivness
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The Turn Algorithm2D example (mesh)
Another possibility
Allows adaptivness but
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The Turn Algorithm2D example (mesh)
Doesnt prevent deadlock
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The Turn Algorithm2D example (mesh)

• West-first
• Prohibit all turns to the west.
• Packet going west must do so before any turn is
  made

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The Turn Algorithm

• In the same way, can be implemented as
• North-last

• - Negative-first

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Chien and Kims algorithm

• Supports adaptivity at the 2-dimensional level
• Uses Virtual channels
• Fault tolerant
• Deadlock preventive

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Virtual Channels

• Virtual channel consists of a buffer that can
  hold one or more flits of a pkt and associated
  state information
• Several VCs share the bw of a single physical
  channel
• Virtual channels decouple allocation of buffers
  from allocation of channels by providing multiple
  buffers for each channel in the network

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Virtual Channels

• Used to avoid blocking (and by extension
  deadlocks)

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Chien and Kims algorithm

• Uses 3 virtual channels per physical channels

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Chien and Kims algorithm

• Divide k-ary n-cube into n-1 adaptive planes
• Divide each plane in 2 virtual networks
  (increasing and decreasing)

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Chien and Kims algorithm

• In the Plane
• As each plane is divided in 2 virtual networks,
  traffic flows in only one direction in the plane
• Outside the planes
• traffic is dimension ordered
• across dimensions.

Deadlock free
Deadlock free
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Fault Tolerance