© KLMH Lienig Multi-Threaded Collision Aware Global Routing Bounded Length Maze Routing

© KLMH Lienig Contributions  Optimal Bounded Length Maze Routing  Heuristic Bounded Length Maze Routing  Parallel Multi-Threaded Collision Aware Strategy for Multi-core Platforms

© KLMH Lienig Bounded Length vs Bounded Box Bounded Length  Algo : 1.While(!viol) { 2. viol = Route(net, BL); 3. if(!viol) { 4. increase BL; 5. } 6. } Start with Manhattan distance Bounded Box  Algo : 1.While(!viol) { 2. viol = Route(net, BB); 3. if(!viol) { 4. increase BB; 5. } 6. } Start with the MBB

© KLMH Lienig Maze Routing Dijkstra Algorithm (G, E, s, t) 1.curr_node = s; 2.Parent[u] = NULL; // parent of all nodes is null initially 3.Q : Fibonacci heap with no nodes initially; //might have multiple copies of a node 4.cost[s] = 0; 5.cost[u] = infinity for all nodes except s 6.While(curr_node != t) { 7. for (each neighbour u of curr_node) { 8. if (cost[curr_node] + w(curr_node, u) < cost[u]) { 9. parent[u] = curr_node; 10. cost[u] = cost[curr_node] + w (curr_node, u); 11. insert_to_Q(u); 12. } 13. } 14. curr_node = Extract_min(Q); //extracts the min cost node from Q 15. } Complexity : O(|E| + |V|log|V|)

© KLMH Lienig PROPOSED ROUTER FLOW DIAGRAM 1.MST Decomposition 2.Congestion Graph G = Route(net, viol) 3.While(!viol) { 4. NCRRoute(net, BL); 5. if(!viol) { 6. BL = Relax(BL) 7. } 8. } 9.Post Refinement 10.Layer Assignment NCR Route : Negotiation Based Congestion Rip up & Route

© KLMH Lienig Optimal Bounded Length Maze Routing  Idea : Discard a path P i (s,v) if, wl(P i ) + Manh(v,t) > BL  Comparison to Traditional Routing: 1.Prunes all paths with BL violations, thereby making the search space smaller 2.Keep more than one path unlike Traditional routing.

© KLMH Lienig OPTIMAL BLMR Cont’d  What happens if keep the paths with lower cost. In this figure, cost(P 1 ) = 80, cost(P 2 ) = 90 wl(P 1 ) = 11, wl(P 2 ) = 5 BL = 16 If we keep only P 1 (lower cost), then it does not have enough slack to detour the congested graph around t. Thus, keep both P 1 & P 2. However, if cost(P i ) < cost(P j ) and wl(P i ) < wl(P j ), then P j is inferior to P j, can discard P j.

© KLMH Lienig Heuristic BLMR Problem with Optimal BLMR  May have any number of paths that meet the criteria. Thus, slower Solution  Need a Heuristic to select a single path.  Examine each path if it has the required wire-length  Select the lowest cost path with enough slack.  If no candidate path have enough slack, select shortest path.

© KLMH Lienig Heuristic BLMR cont. Heuristic : ew k (v,t) = Manh(v, t) × (L k-1 (s, t) / Manh(s, t)) --1 Condition : wl(P i )+ ew k (v, t) ≤ BL ew k (v,t) : estimated wire length from v to t in kth iteration L k-1 (s, t) : actual routed wirelength from s to t in k-1th iteration Pi(s,v) : Path from s to v wl(P i ) : wirelength of Path Pi Manh(v, t) : Manhattan distance from v to t Manh(s, t) : Manhattan distance from s to t The heuristic keeps on getting better with each iteration : 1.Overestimated wl from v to t in k th iteration : Path might be discarded by equation 2, thus in (k+1) th iteration, it gets better. 2.Under-estimated wl from v to t in k th iteration, actual wl (L K ) corrects it in the next iteration

© KLMH Lienig Bounded Length Relaxation  With each iteration of rip-up & re-route, 1.Overflow decreases 2.Wire-length increases For the nets to be routed in the next iteration, BL is relaxed BL nk = Manh (s n, t n ) × (arctan(k − α ) + β) α, β are user defined (use 9,2.5 for this paper)

© KLMH Lienig Task-Based Concurrency Parallelism  Rip & Re-route still takes 99.6% of total routing time on one of the difficult benchmarks (ISPD2007) Task Based vs Partition Based Concurrency  Load might not be shared evenly between the threads because of differing congestions in different parts of the grid graph.

© KLMH Lienig Partition vs TCS TASK BASED CONCURRENCY & CHALLANGES  Each entry in Task Q is a 2 pin routing task  All threads pull one task out of Q Issues  Same routing resource can be used by two threads unaware of each other.  No Common Routing Resources (search restricted by partition)

© KLMH Lienig Challenges Cont’d

© KLMH Lienig Maze Routing & Collisions Maze routing happens in two phases : 1.Wave Propagation : explore every possible move. 2.Back-Tracing : Identify new routing path based on the paths explored. When will it be clear that collision occurred ?  Not clear at Wave propagation  Not clear at BackTracing  Clear after BackTracing – both the threads have used the resource.

© KLMH Lienig Collision Aware RR Observations : 1.Nets closer are the most likely candidates for collisions. 2.About 41% of overflow nets in RR are due to collisions. 3.An overflow net has few overflow edges 4.It reuses most of its edges (80% of edges re-used)

© KLMH Lienig Using Observations in Collision Aware RR  Thread T2 : marks the edges previously used by the net  Thread T1 : see the increased cost of the common edges

© KLMH Lienig ALGORITHMS Collision Aware RR 1.Algorithm Collision-aware Rip-up and Reroute 2.Input: grid graph G 3.TaskQueue TQ; 4.while ( G has overflows) 5.update(TQ) // insert overflow net to TQ; 6.//parallel by each thread 7.while (TQ is not empty); 8.N ←extract_a_task(TQ); 9. BL k n ←relax_bounded_length(N); 10.collision_aware_BLMR(G, N, BL k n ); 11.end while end Collision Aware BLMR 1.Algorithm Collision-aware BLMR 2.Input: grid graph G, net N, bounded-length BL 3.mark_grid_edge( path(N),G); 4.ripup(path(N),G); 5.collision_aware_wave_propagation( N, G, BL); 6.newPath←back_tracing(N, G); 7.unmark_grid_edge(path(N),G); 8.path(N)←newPath; end

© KLMH Lienig Evaluation

© KLMH Lienig Evaluation cont.

© KLMH Lienig Summary BLMR  Bounded length Maze Routing  Optimal BLMR : paths based on slack left to reach the target  Heuristic BLMR : select a single path based on heuristic  The heuristic gets better with each iteration of rip-up & reroute Task Based Concurrency  Better for load sharing compared to partition based concurrancy  Collision may occur due to same resource used by more than one thread  Collision Aware RR : avoid overflow due to race conditions.

© KLMH Lienig THANK YOU