Congestion minimization during placement

Typical placement objectives involve reducing net-cut cost or minimizing wirelength. Congestion minimization is the least understood, however, it models routability most accurately. In this paper, we study the congestion minimization problem during placement. First, we show that a global placement with minimum wirelength has minimum total congestion. We show that minimizing wirelength may (and in general, will) create locally congested regions. We test seven different congestion minimization objectives. We also propose a post processing stage to minimize congestion. Our main contribution and results can be summarized as follows. (1) Among a variety of cost functions and methods for congestion minimization (including several currently used in industry), wirelength alone followed by a post processing congestion minimization works the best and is one of the fastest. (2) Cost functions such as a hybrid length plus congestion (commonly believed to be very effective) do not always work very well. (3) Net-centric post-processing techniques are among the best congestion alleviation approaches. (4) Congestion at the global placement level, correlates well with congestion of detailed placement.