The problem looks like a form of bin packing (and possibly job shop scheduling), which are NP-complete, so OptaPlanner will do better than a FIFO algorithm.
But is it really NP-complete? If all of these conditions are met, it might not be:
- All 40 servers are identical. So running a priority report on server A instead of server B won't deliver a report faster.
- All 40 servers are identical. So total duration (for a specific input set) is a constant.
- Total makespan doesn't matter. So given 20 small jobs of 1 hour and 1 big job of 20 hours and 2 machines, it's fine that it takes all small jobs are done after 10 hours before the big job starts, given a total makespan of 30 hours. There's no desire to reduce the makespan to 20 hours.
- "the average time in the queue" is debatable: do you care about how long the jobs are in the queue until they are started or until they are finished? If the total duration is a constant, this can be done by merely FIFO'ing the small jobs first or last (while still respecting priority of course).
- There are no dependencies between jobs.
If all these conditions are met, OptaPlanner won't be able to do better than a correctly written greedy algorithm (which schedules the highest priority job that is the smallest/largest first). If any of these conditions aren't met (for example you buy 10 new servers which are faster), then OptaPlanner can do better. You just have to evaluate if it's worth spending 1 thread to figure that out.
If you use OptaPlanner, definitely take a look at real-time scheduling and daemon mode, to replan as new reports enter the system.
