1. Placement and Routing With Congestion Control
ECE556 Final Project
Harish Krishnan

2. Placement and Routing Placement - Simulated Annealing.
Routing - A Rectilinear Steiner Tree to connect the pins.
Both the above methods are extremely popular and widely implemented.

3. Congestion Control What is the need for congestion control ?
Prevent spilling of routes into channels. Excessive congestion results in local shortage of routing resource.
The Problem :- So how do we model congestion ?
We need to find the parameters that affect congestion so as to arrive at an objective function to minimize.

4. Congestion Control Our Model of Congestion – ‘Bin’ based approach
Global Bins
Cells
Global Edges
Congestion is quantified as number of crossings between routed nets and Global
Bin edges

5. Congestion Model
For each global edge ‘e’ , de, the Routing Demand of ‘e’ is defined as the number of nets crossing ‘e’.
The Routing Supply of edge ‘e’, se is a fixed value and a function of the edge length and technology parameters
A global edge ‘e’ is congested if and only if the routing demand exceeds the routing supply (de > se)
The overflow of edge ‘e’ in case it is congested is the difference between the demand and supply . We shall use the congestion overflow as the objective to minimize.
The congestion overflow of the layout is defined as the summation of the overflow for all global edges.

6. Wire length Minimization vs. Congestion Minimization
Minimizing wire length is equivalent to minimizing average congestion. However wire length and congestion can be inconsistent in local regions.
Congestion Minimized.
No overflow.
Wire length minimized. However there is
overflow

7. Post Processing Congestion Reduction Algorithms
Flow – based cell centric algorithm :- Uses a flow based approach to move multiple cells simultaneously. Attempt to find better location for cells so as to minimize congestion. Can be modeled easily as a Transportation Problem where the source are the cells and the destinations are the global bins.
Problem – Huge Memory Requirements.
Program has a tendency to crash in case of large sample cases.

8. Post Processing Algorithm
Net – Centric Algorithm :- Tries to move net one by one so as to reduce congestion.
Let us assume we have a placement for the given cells.
Route all the nets.
Compute weight for each net. The weight of each net is the number of overflowed global edges the net crosses.
Sort the nets in descending order according to their weights.
Move net so as to minimize the congestion overflow. Repeat Step 4 until all the nets are analyzed.
Update net weights.
Repeat above steps until congestion overflow cannot be further minimized. ( We may also set a threshold if we find that the algorithm takes too long to complete).
Thus the entire procedure is a ‘rip and reroute’ implementation.

9. Credits Modeling and minimization of routing congestion.
Maogang Wang and Majid Sarrafzadeh, North Western University
On the Behavior of Congestion minimization during placement
Wang, Sarrafzadeh, North Western University
An efficient algorithm for VLSI global routing
Shawki Areibi , Min Xie and Anthony Vannelli