1. 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

2. 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

3. 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

4. 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?

5. 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)

6. 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)

7. 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

8. 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

9. 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)

10. 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

11. 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

12. 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

13. 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

14. 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

15. 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

16. 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

17. 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

18. 18 Evgeny Bolotin – Efficient Routing, DATE 2007 Scaling of Savings : DR and SR Savings Network Size

19. 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

20. 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)