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1 Evgeny Bolotin – Efficient Routing, DATE 2007 Routing Table Minimization for Irregular Mesh NoCs Evgeny Bolotin, Israel Cidon, Ran Ginosar, Avinoam Kolodny QNoC Research Group Technion Electrical Engineering Department Technion, Haifa, Israel
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2 Evgeny Bolotin – Efficient Routing, DATE 2007 Motivation Generic NoC: Mesh Topology Low Cost Links Shortest Path (SP) Static Function Routing Low Cost Router Power-Efficient Communication Low Cost Interfaces (no reordering) Example: XY routing Practical System Practical NoC (QNoC and others) Custom/Irregular Low Cost
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3 Evgeny Bolotin – Efficient Routing, DATE 2007 Routing in Irregular Mesh NoC The Problem: Simple Function (e.g. XY) is not working Around the Block Dead End
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4 Evgeny Bolotin – Efficient Routing, DATE 2007 Traditional Approach Traditional Irregular Routing Distributed Routing – Tables in Routers Output Port for All Destinations Size of Tables ~ Source Routing – Tables in Sources List of routing tags (for each hop) for All Destinations Size of Tables ~ Large Area/Power Cost Increased Access Time Can we do better than full routing tables?
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5 Evgeny Bolotin – Efficient Routing, DATE 2007 Low Cost Routing Solution: 1. Don’t Use Full Routing Tables (Customize) Store only relevant destinations Routing Table Cost : Area [gates] ≈ Sizof(Entries) + Sizof (Lookup Logic) Power ≈ Const * Area 2.Minimize Routing Table Size: Routing Function + Reduced Routing Table ( for deviations only)
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6 Evgeny Bolotin – Efficient Routing, DATE 2007 Minimize Table Entries : Distributed Routing: Default Function (“Go XY” or “Don’t turn”) Reduced Routing Tables (for deviations) No Entries Routing Entry “Don’t Turn” – Turn-Table (TT) No EntriesRouting Entry “Go XY” – XY Deviations Table (XYDT)
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7 Evgeny Bolotin – Efficient Routing, DATE 2007 Minimize Table Entries : Source Routing: Default Function (“Go XY” or “Don’t turn”) Reduced routing-tag list (only for deviations) Source Routing for Deviation Points (SRDP) Example : Specific routers are Deviation Points Default function routing in other routers
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8 Evgeny Bolotin – Efficient Routing, DATE 2007 Path Selection Algorithms Build Auxiliary Graph: Turns-Graph ‘K’- Large Number, price of “Hop” ‘1’- Price of “Turn” ‘0’- Price of “Don’t Turn” Apply Shortest Path (Dijkstra) on Turns-Graph: Primary objective: minimal number of hops Secondary objective: minimal number of turns Shortest Path with Minimal Number of Turns: OriginalTurns-Graph
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9 Evgeny Bolotin – Efficient Routing, DATE 2007 Path Selection Algorithms Turn-Table (TT) Algorithm TT Idea: Combine routing paths from different sources Among all shortest paths between all S-D pairs, choose covering set of paths that minimize the total number of entries in the network Turns-Tables. Question: Is it Shortest Path with Minimal Number of Turns? Scenario AScenario B Answer: No! Example: Choose: Scenario A: Green path makes 3 Turns Scenario B: Green path makes 2 Turns A: Green doesn’t add new entries (uses existing)
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10 Evgeny Bolotin – Efficient Routing, DATE 2007 Turn-Table (TT) Algorithm TT Algorithm (greedy) – Informal Explanation: Iteration #1 Iteration #2 Calc Distances Choose Closest Src Create Path to D Update Distances Choose Closest Src Create Path to D In each iteration choose a shortest path from closest Src to D, or to an already existing path to D, which adds the minimal number of turns
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11 Evgeny Bolotin – Efficient Routing, DATE 2007 XY-Deviation Table (XYDT) XYDT Problem definition: Among all shortest paths between each S-D pair, select a path that minimizes number of routing steps which deviate from XY routing policy. XYDT Algorithm Outline: For each destination D: Sort nodes according to distance from D For nodes at same distance from D: Among all shortest-path steps choose XY step if exists Repeat until paths from all sources are found
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12 Evgeny Bolotin – Efficient Routing, DATE 2007 Further Minimization Tricks Turn-Table Routing: At source router every step is turn Default Direction at the source router XY-Deviation Table Routing: Save entry if XY-port is missing and routing goes YX No Entries Routin g Entry
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13 Evgeny Bolotin – Efficient Routing, DATE 2007 Source Routing for Deviation Points (SRDP) The Idea : Specific routers are Deviation Points Default function for all other routers SRDP Algorithm Outline: Find paths using algorithms above Find deviation points (DPs) Calculate Reduced SRDP headers
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14 Evgeny Bolotin – Efficient Routing, DATE 2007 Evaluation Method Efficient Routing Path Selection Algorithms Distributed/Source Routing “XY”/ “ Don’t Turn” Irregular grid Random holes insertion Inter-module Connections Matrix Random hotspots insertion oConnection to hotspot (high probability) oConnection to non-hotspot (low probability) Real examples - Video Processing Apps (MPEG, VOPD) Compare Cost [gate count] Tables containing relevant destination only Our method : Routing function + Table for deviations
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15 Evgeny Bolotin – Efficient Routing, DATE 2007 Results - Typical NoC Distributed Routing“Turns Table” 3X “XY Deviations Table” 8X Source Routing“SR Deviation Points” 2.5X Savings in small system with high irregularity and few destinations
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16 Evgeny Bolotin – Efficient Routing, DATE 2007 Results - Typical NoC Distributed Routing“Turns Table” 3.7X “XY Deviations Table” 34X Source Routing“SR Deviation Points” 2X Savings in large system with low irregularity and many destinations
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17 Evgeny Bolotin – Efficient Routing, DATE 2007 Results –Video Apps Distributed Routing“Turns Table” 4.4X “XY Deviations Table” 40X Source Routing“SR Deviation Points” 2.9X Savings in video processing app. system
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18 Evgeny Bolotin – Efficient Routing, DATE 2007 Scaling of Savings : DR and SR Savings Network Size
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19 Evgeny Bolotin – Efficient Routing, DATE 2007 Scaling: Source vs. Distributed Routing Good Scaling with Distributed Routing Best Scaling with XYDT Large system with low irregularity and many destinations
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20 Evgeny Bolotin – Efficient Routing, DATE 2007 Summary The Problem – Low Cost Routing In Irregular Mesh Idea: Combine Default Function + Reduced Deviation Tables Distributed Routing and Source Routing Path Selection Algorithms Shortest path + Minimum Total Table Entries Numerical Results Significant Cost Reduction – DR(40X); SR(3X) Scalability of Savings Good Scalability of Distributed Routing (XYDT the best)
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