I gave a talk at ROADEF2018 about using general-purpose solvers for large-scale partitioning problems. Although much slower than ad-hoc algorithms, they can still converge to a good solution with some help… and they can tackle much more complex models.
The choice between analytical placement and partitioning is a crucial and early one. For Coloquinte, however, I made this choice a long time ago.
There are two obvious reasons: analytical placement generally performs a little better and I know it better.
However, there are deeper reasons why I want to commit to using analytical placement. In my opinion, analytical placement yields better flexibility and modularity.
First, it is tunable with simple but numerous parameters. By changing the cost function, you can in theory optimize for timing and power. The spreading forces control the tradeoff between speed of convergence and solution quality.
With partitioning-based placement, you need to design whole new algorithms for power, timing and congestion optimization (which many papers did). With a complete analytical placer, you can test parameter tuning within minutes: change the cost function and the spreading schedule, then execute. And there are a lot of cost functions to chose from.
Another important property of some analytical placement algorithms is that they modify an existing placement at each step, not build it from scratch. For this reason, it is convenient to perform netlist or placement changes. Whether to interface it with other programs (performing cell resizing and buffering for example) or to enable engineering change orders (ECOs), it is a useful feature. I suppose it makes it easier to adapt the placer to new problems, with all the performance-critical code factored between the solver and the legalizer.
Therefore, I am going to use an analytical placement that continuously spreads the cells: if region constraints are used, circuit modifications are not as easy. Additionally, this method is the leading one on benchmarks: using it should improve both flexibility and efficiency.
That doesn’t mean writing a complete placement tool is easy: external tools are needed to analyze timing and congestion, but with analytical placement the interface may be very loose and quite modular. I am going to show Coloquinte’s interface in the next post.