Performance and Power Efficient OnChip Communication Using Adaptive Virtual PointtoPoint Connections

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Performance and Power Efficient On-Chip
Communication Using Adaptive Virtual
Point-to-Point Connections

• M. Modarressi, H. Sarbazi-Azad, and A. Tavakkol
• Computer Engineering Department, Sharif
  University of Technology, Tehran, Iran
• modarressi_at_ce.sharif.edu

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Outline

• Introduction and Motivations
• Virtual Point-to-Point (VIP) Connections
• Static VIP Construction Scheme
• Dynamic VIP Construction Scheme
• Setup Network
• Evaluation Results
• Conclusions and Future Work

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Sharif University of Technology
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On-Chip Communication Mechanisms

• Packet-Switched NoCs
• Good Resource Utilization
• Modest Design Effort/Time Due to Structured and
  Predictable Links
• Some Power and Performance Overheads Due to
  Multi-Stage Pipelined Routers
• Dedicated Point-to-Point Links
• Ideal Power and Performance
• Poor Scalability Significant Area Overhead for
  Large Systems
• Significant Design Effort/Time Due to
  Non-Predictable Link Properties

?Virtual Point-to-Point Connections in a
Packet-Switched NoC
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VIP Connections

• VIP VIrtual Point-to-point Connections
• Over One VC (Virtual Channel) of Each Physical
  Channel
• Bypass Some Router Pipeline Stages
• Inexpensive Extensions to a Traditional Wormhole
  Router
• Router Control Unit, Arbiter, Buffer of the VIP
  Virtual Channels

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Router Architecture

• Buffer at the VIP Virtual Channels Is Replaced by
  a Register (1-Flit Buffer)

• VIP Paths Are Kept by VIP Allocator Units at
  Output Ports
• ?Determines Which Input Is Connected to This
  Port Along the VIP

• Allocates Output Port to VIP When Control Signals
  Indicate That the VIP Has an Incoming Flit to
  Forward
• ?A Flow-Control Mechanism Prevents Starvation in
  Packet-Switched Flits

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VIP Connections

• A VIP Is Constructed by Chaining the VIP
  Registers in the Routers Between the Source And
  Destination Nodes of a Communication Flow
• Provides a Virtual Dedicated Pipelined Link With
  1-flit VIP Buffers as Staging Registers
• Flits Only Travel Over the Crossbars and Links
  Which Cover the Actual Physical Distance Between
  Their Source and Destination Nodes
• Skip Through Buffer Read, Buffer Write, and
  Allocation Operations

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VIP Connections

• VIPs Are Not Allowed to Share a Common Link
• To Remove Buffering, Arbitration,
• A Limited Number of VIPs in a Network
• But VIPs Cover a Significant Portion of On-Chip
  Traffic Due to Communication Locality
• In Most Multi-Core SoC Applications Each Core
  Communicates With a Few Other Cores
• In CMP Workloads Each Node Tends to Have a Small
  Number of Favored Destinations for Its Messages

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VIP Construction Algorithm - Static

• Based on Application Traffic Pattern
• Input Applications Are Described by a Task-Graph
  (TG)
• A Heuristic Algorithm
• Map the TG Cores into the Nodes of a Mesh-based
  NoC
• Construct VIP for TG Edges in Order of Their
  Communication Volumes
• Find a Path Through Packet-Switched Network for a
  TG Edge If There Are Not Sufficient Free
  Resources to Build a VIP for It

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VIPs for the VOPD Application

• VIPs Cover 100 of the On-Chip Traffic for This
  Application
• Static VIP Construction Scheme
• Benchmarks VOPD, MWD, MPEG, MP3H263
• Up to 58 Reduction in Message Latency (39 on
  Average)
• Up to 65 Reduction in Power Consumption (49 on
  Average)

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VIPs vs. Physical Point-to-Point Connections

• VIPs Offer
• Power and Performance Close to Dedicated Physical
  Point-to-Point Connections
• More Flexibility
• Dynamically Reconfigurable Based on the Traffic
  Pattern of the Running Application
• Less Design Effort
• Customized Dedicated Connections Over Regular
  Components

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Dynamic VIP Construction

• An Alternative VIP Construction Scheme
• Dynamically Changes the VIP Connections in
  Response to Communication Requirements Imposed By
  the Running Application
• Monitoring the NoC Traffic
• Detecting High-Volume Communications and
  Constructing a VIP for Them
• Select the Best Route for a VIP Using a Simple
  Setup Network

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Setup Network

• Setup Network Structure
• A Light-Weight Control Network
• Simple Node Structure and Small Bit-Width
• The Same Topology as the Main Data Network
• Setup Network Operation
• Keep the Track of the Number and Destination of
  Packets Sent by Each Node
• Select Traffic Flows Weighting Higher Than a
  Threshold (Bit/Sec.)
• Finds a Path Along One of the Shortest Paths
  Between the Source and Destination Nodes of the
  Traffic Flow to Construct a VIP

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Dynamic VIP Construction

• Establishing a New VIP May Tear Down Some
  Existing VIPs
• Cost of a VIP The Cumulative Weight (bit/sec.)
  of the VIPs That Will Be Torn Down By This New
  VIP
• Setup Network
• Finds the Path With Minimum Cost
• Sends the Cost to the Source Node to Decide on
  Establishing the New VIP
• A New VIP Is Established If the Cumulative Weight
  of the Torn Down VIPs Is Less Than the Weight of
  the Requesting Traffic Flow

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Setup Network

• VIP Setup Procedure
• Arbitrating Among VIP Setup Requests
• Running the Distributed VIP Setup Algorithm
• Setting Up a VIP in the Data Network By
  Configuring the VIP Allocator of the Nodes Along
  the VIP Path
• Tearing Down Conflicting VIPs
• Each Setup Network Node Contains the
  Configuration Information of Its Corresponding
  Data Network Node
• Due to the Distributed Nature of the Algorithm
• ?Short Reconfiguration Time

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Select the Minimum Cost and Keep the Port from
Which the Smaller Cost Is Received
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9
2
10
9
15
4
5
7
5
8
0
3
5
9
12
1. Add the Received Cost (4) to the Weight of
Ports Along the Shortest Path (the W and N Ports)
toward the Destination Node 2. Send the New
Costs (9 and 12) to the Neighboring Nodes Along
the Destination Node
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0
5
4
12
4
8
Port Cost ( Weight of the VIP Using It )
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Dynamic VIP Construction

• The Setup Network Operates in Parallel with
  Packet Transmission in Packet-switched Network
• Hide the Setup Time
• The Setup Network Has a Small Bit-width and
  Operates Infrequently (Only When a High-volume
  Flow Is Detected)
• Negligible Power and Area Overhead

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Evaluation Results

• XMulator NoC Simulator (www.xmulator.org)
• A C -based Simulator
• Orion Power Library
• Comparison with a Conventional NoC (5-Stage
  Pipelined Wormhole Switch)
• Multi-Core SoC Traffic
• H.263 DecoderMP3 Decoder, H.263 Decoder MP3
  Encoder, MP3 Decoder MP3 Encoder
• ?38 Reduction in Message Latency, 46
  Reduction in Power Consumption

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Evaluation Results

• Synthetic Traffic
• N-Hot Traffic 80 of Messages to Exactly N
  Destination, 20 to Randomly Chosen Nodes

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Summary and Future Work

• Adaptable Virtual Point-to-Point Connections in a
  Packet-Switched NoC
• Benefit from the Advantages of Both Communication
  Methods
• Two Static and Dynamic VIP Construction Schemes
• Significant Power/Latency Reduction
• Future Work
• Comparing the Method with Related Work Express
  Virtual Channels, Single-Cycle Routers,
• Precise Area/Power Results by Implementing the
  NoC in Hardware
• Analytical Models Show Small Area Overhead

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Thank You

• Questions?
• modarressi_at_ce.sharif.edu

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