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Non-minimal Routing. Non-minimal routing Wormhole degrades performance while VCT has less secondary effects Fault tolerance is the main motivator Classes Search-based algorithms Virtual channel-based routing Turn-based routing. Non-Minimal Routing . Reading. Section 4.7 and/or

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Non-minimal Routing


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non minimal routing
Non-minimal Routing
  • Non-minimal routing
    • Wormhole degrades performance while VCT has less secondary effects
    • Fault tolerance is the main motivator
  • Classes
    • Search-based algorithms
    • Virtual channel-based routing
    • Turn-based routing

Non-Minimal Routing

reading
Reading
  • Section 4.7 and/or
  • P.T. Gaughan, et al., “Distributed, deadlock-free routing in faulty, pipelined, direct interconnection networks,” IEEE Transactions on Computers, vol. 45, no. 6, pp.651-665, June 1996
  • A. Mejia , J. Flich, J. Duato, Sven-Arne Reinomo and Tor Skeie, “Segment Based Routing: An Efficient Fault-Tolerant Routing Algorithm for Meshes and Tori,” Proceedings of the International Parallel and Distributed Processing Symposium, April 2006
  • From J. Flich, A. Mejia, P. Lopez, and J. Duato, “Region-Based Routing: An Efficient Routing Mechanism to Tackle Unreliable Hardware in Networks on Chip,” Proceedings of the First International Symposium on Networks on Chip, May 2007
backtracking protocols
Backtracking Protocols
  • Backtracking search + resource reservation
  • Constrain the search
    • Minimal paths vs. #misroutes

P.T. Gaughan, et al., “Distributed, deadlock-free routing in faulty, pipelined, direct interconnection networks,” IEEE Transactions on Computers, vol. 45, no. 6, pp.651-665, June 1996

Non-Minimal Routing

optimization
Optimization
  • Sensitive to choice of switching technique
    • Naturally suited to circuit switching and pipelined circuit switching
    • Overhead is large with SAF
  • Deadlock is avoided by not blocking on busy channels
  • Livelock is avoided by maintaining and using search history
    • In the header: large headers
    • In the routers: local state, headers comparable to e-cube
  • Protocol variations
    • Multi-links
    • k-family
    • exhaustive: profitable and misrouting
    • limited misrouting
    • multi-phase

Non-Minimal Routing

topology agnostic routing
Topology Agnostic Routing
  • Topology dependent vs. topology agnostic routing
    • Reliability
    • Increasingly important on-chip
  • Approaches
    • Techniques based on virtual channels
      • Expensive on-chip
      • Competes with QoS schemes
    • Techniques based on Turn restrictions
      • Difficult to ensure non-minimal paths

Topology Agnostic Routing

segment based routing
Segment Based Routing
  • Topology agnostic routing
  • Restriction-based approach
    • Multiple restriction options
      • Select restrictions based on performance goals
    • Source based routing
      • Routing table generation

From A. Mejia , J. Flich, J. Duato, Sven-Arne Reinomo and Tor Skeie, “Segment Based Routing: An Efficient Fault-Tolerant Routing Algorithm for Meshes and Tori,” Proceedings of the International Parallel and Distributed Processing Symposium, April 2006.

Topology Agnostic Routing

key idea segments subnets
Key Idea: Segments & Subnets
  • Partition topology into subnets and then segments in a subnet
  • Goal: islands of regularity

Topology Agnostic Routing

key idea optimization
Key Idea: Optimization
  • Placement of Turn restrictions in a segment
    • Placement for latency  shortest path
    • Placement for throughput  distribute traffic
  • Topology segmentation
    • Can be optimized for regular topologies

Topology Agnostic Routing

requirements
Requirements
  • Avoid deadlock in a segment
  • Avoid deadlock when traversing multiple segments
  • Ensure routing connectivity when physical connectivity exists
  • Avoiding congestion in path construction

Topology Agnostic Routing

construction of segments
Construction of Segments

Starting node

  • Search for starting segment + “regular” segments
    • Unitary segments
  • Add one bidirectional restriction in each segment

Failed links

Terminal node

Unitary segment

bridge segment

Topology Agnostic Routing

segment types
Segment Types

Starting node

  • Starting segment
  • Regular segment
  • Unitary segment

Failed links

Terminal node

Unitary segment

bridge segment

Topology Agnostic Routing

deadlock freedom
Deadlock Freedom
  • One routing restriction per segment
    • No cycles in a segment
  • Every cycle contains a segment
    • Hence cannot be “closed” to create deadlock
  • No cycle from the start node back to itself
    • Cannot create cycles across subnets
  • Think of a subnet as a union of 1-D segments

Topology Agnostic Routing

example
Example

From A. Mejia , J. Flich, J. Duato, “On The Potential of Segment Based Routing” Proceedings of the International Conference on Parallel Processing 2008

routing
Routing
  • Segment routing is turn based and therefore partially adaptive
  • Source routing can be layered on top of segments to balance traffic

From A. Mejia , J. Flich, J. Duato, “On The Potential of Segment Based Routing” Proceedings of the International Conference on Parallel Processing 2008

performance
Performance

From A. Mejia , J. Flich, J. Duato, Sven-Arne Reinomo and Tor Skeie, “Segment Based Routing: An Efficient Fault-Tolerant Routing Algorithm for Meshes and Tori,” Proceedings of the International Parallel and Distributed Processing Symposium, April 2006.

Topology Agnostic Routing

region based routing

S

D

Region-Based Routing
  • Recognize that routing decisions implicitly check for region membership
    • Think meshes
  • Generalize the idea of regions
    • Can naturally be adapted for fault tolerant routing

From J. Flich, A. Mejia, P. Lopez, and J. Duato, “Region-Based Routing: An Efficient Routing Mechanism to Tackle Unreliable Hardware in Networks on Chip,” Proceedings of the First International Symposium on Networks on Chip, May 2007

Topology Agnostic Routing

example of regions
Example of Regions
  • Static, off-line topology characterization
  • Online querying of network structure
    • Built on segment-based routin

{node set}

{node set}

From J. Flich, A. Mejia, P. Lopez, and J. Duato, “Region-Based Routing: An Efficient Routing Mechanism to Tackle Unreliable Hardware in Networks on Chip,” Proceedings of the First International Symposium on Networks on Chip, May 2007

Topology Agnostic Routing

example of regions1
Example of Regions
  • What are the characteristics of these regions?
    • Note #regions = f(routing options)
  • Note use of output port depends on input port
    • Check W output port from N input port

{node set}

{node set}

From J. Flich, A. Mejia, P. Lopez, and J. Duato, “Region-Based Routing: An Efficient Routing Mechanism to Tackle Unreliable Hardware in Networks on Chip,” Proceedings of the First International Symposium on Networks on Chip, May 2007

Topology Agnostic Routing

approach
Approach
  • Observe that table-based routing is really region based
    • Each entry identifies a region
  • Merge entries into compact region specifications at each switch
  • Region construction is based on the paths
    • Any set of paths  fault tolerant routing
    • No virtual channels

Topology Agnostic Routing

key idea
Key Idea
  • Generate paths
    • All minimal
    • First non-minimal
    • Note: using SR routing
  • Record paths at each router
    • Produce region representation for each output port
    • Record input port dependencies
  • Program Routers

From J. Flich, A. Mejia, P. Lopez, and J. Duato, “Region-Based Routing: An Efficient Routing Mechanism to Tackle Unreliable Hardware in Networks on Chip,” Proceedings of the First International Symposium on Networks on Chip, May 2007

Topology Agnostic Routing

creating regions
Coalesce routing options based on inputs and outputs

Represents a compact routing table

Creating Regions

From J. Flich, A. Mejia, P. Lopez, and J. Duato, “Region-Based Routing: An Efficient Routing Mechanism to Tackle Unreliable Hardware in Networks on Chip,” Proceedings of the First International Symposium on Networks on Chip, May 2007

Topology Agnostic Routing

region construction
Region Construction

Segment Routing

@each node

@each node

Search

Coalesce & Packing

Region Formation

  • Note this can be applied to any topology
  • No virtual channels
  • Offline optimization of latency vs. distance

From J. Flich, A. Mejia, P. Lopez, and J. Duato, “Region-Based Routing: An Efficient Routing Mechanism to Tackle Unreliable Hardware in Networks on Chip,” Proceedings of the First International Symposium on Networks on Chip, May 2007

Topology Agnostic Routing

hardware overheads
Hardware Overheads
  • Each region requires
    • Four registers that define the region
    • Mask registers to define input and output ports
    • Logic to determine routing options
  • Hardware cost grows as the number of regions
    • Growth as f(network_size) is much slower

From J. Flich, A. Mejia, P. Lopez, and J. Duato, “Region-Based Routing: An Efficient Routing Mechanism to Tackle Unreliable Hardware in Networks on Chip,” Proceedings of the First International Symposium on Networks on Chip, May 2007

Topology Agnostic Routing

implementation
Implementation
  • Initialization of region registers and parallel evaluation of all regions

From J. Flich, A. Mejia, P. Lopez, and J. Duato, “Region-Based Routing: An Efficient Routing Mechanism to Tackle Unreliable Hardware in Networks on Chip,” Proceedings of the First International Symposium on Networks on Chip, May 2007

Topology Agnostic Routing

microarchitecture issues
Microarchitecture Issues
  • Routing algorithm performance is sensitive to resource allocation schemes in the router
  • Key resource management functions include
    • Routing function
    • Selection function
    • Arbitration/scheduling
  • Mismatch can lead to poor performance
resource allocation selection functions
Resource Allocation: Selection Functions

VC status/control

  • Selection function may be oblivious or informed
    • Common to favor minimal paths and lightly loaded links
  • Examples:
    • Meshes: minimum congestion, maximum flexibility, straight lines
  • Unlike routing functions, selection function must be serialized
    • Result updates the channel status – a centralized resource

Routing function

VC buffer

Selection function

Input VC

Output VCs

selection functions
Selection Functions
  • Favor adaptive channels
    • Improve probability of escape channel availability
    • Time dependent selection functions: give adaptivity a chance
  • Selection functions for real time traffic
    • Separate best effort and guaranteed packets via VCs or virtual networks
  • Note the impact on bisection utilization
  • Selection functions for cache coherent systems?
resource allocation arbitration
Resource Allocation: Arbitration

arbitration

  • Tradeoffs: channel bandwidth vs. message sizes and types
    • Mix of buffering strategies across message types
  • All three strategies must be co-designed for a tuned system

Message size

Flow control

routing selection arbitration
Routing, Selection & Arbitration
  • Input driven vs. output driven scheduling
    • Output driven scheduling requires replication of routers amongst inputs
  • Lessons from the microprocessor world
    • Impact of complexity, workloads, and concurrency
  • Impact of….
    • Symmetry of the topology
    • Locality of traffic
    • Packet size

Locality, uniformity

deterministic routing

adaptive routing

Irregular, hot spot

characterization of techniques
Characterization of Techniques
  • Deadlock freedom achieved by
    • Path based techniques
      • Restrict paths
    • Buffer based techniques
      • Structured buffer pools
    • Channel based techniques
      • #VCs independent of the network
summary
Summary
  • Best routing algorithm driven by multiple considerations

Hot spots

Deterministic vs. adaptive

Packet sizes

Compatible micro-architecture

Locality of traffic

Uniform vs. non-uniform traffic

Power envelope

On-chip vs. off-chip

Symmetric vs. asymmetric topology