Parallel FPGA Router using Sub-Gradient method and Steiner tree

Rohit Agrawal, Chin Hao Hoo, Kapil Ahuja, Akash Kumar

In the FPGA (Field Programmable Gate Arrays) design flow, one of the most time-consuming step is the routing of nets. Therefore, there is a need to accelerate it. In a recent paper by Hoo et. al., the authors have developed a Linear Programming based framework that parallelizes this routing process to achieve significant speedups (the algorithm is termed as ParaLaR). However, this approach has certain weaknesses. Namely, the constraints violation by the solution and a local minima that could be improved. We address these two issues here. In our paper, we use this framework and solve it using the Primal-Dual sub-gradient method that better exploits the problem properties. We also propose a better way to update the size of the step taken by this iterative algorithm. We perform experiments on a set of standard benchmarks, where we show that our algorithm outperforms the standard existing algorithms (VPR and ParaLaR). We achieve up to 22% improvement in the constraints violation and the standard metric of the minimum channel width when compared with ParaLaR (which is same as in VPR). We achieve about 20% savings in another standard metric of the total wire length (when compared with VPR), which is the same as for ParaLaR. Hence, our algorithm achieves minimum value for all the three parameters. Also, the critical path delay for our algorithm is almost same as compared to VPR and ParaLaR. We achieve relative speedups of 3 times when we run a parallel version of our algorithm using 4 threads.

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