What is the bottleneck in my Solver?

"We need faster hardware." It is the typical knee-jerk reaction to any performance problem. However, for most optimization problems, throwing hardware at the problem doesn’t help much (in part because of the size of the search space).

For example, presume that the score of the best solution evolves like this over time:

At any point in time (X axis), it shows the score of the best solution (Y axis) found until then. Notice that if our hardware is twice as fast (so if we need only half the amount of time), we’d get the score we’re currently getting at 2 mins 30 secs, which is about the same as the score we’re getting after 5 minutes. So throwing hardware at the problem would hardly improve the solution. In this case, performance isn’t the bottleneck.

Instead of wildly guessing at the problem, it’s better to configure the Solver in the OptaPlanner Benchmarker and let it generate a useful benchmark report. In that report, look at the best score over time graph (similar to the one shown above) and check if you can see any of these 3 patterns on it:

A JVM profiler (such as JProfiler or VisualVM) can be very helpful in the first case, but not in the other 2 cases.

When facing a performance or scalability challenge, don’t randomly improve parts of your code. Remember that premature optimization is the root of all evil. Instead, let the OptaPlanner Benchmarker guide you and fix the biggest bottlenecks first.