Scheduling of reduce tasks results in starvation

If a job has more reduce tasks than there are containers available, then the reduce tasks can occupy all containers causing starvation. The attached graph illustrates the behaviour. Scheduler used is fifo.

I understand that the correct behaviour when all containers are taken by reducers while mappers are still pending, is for the running reducers to be "pre-empted". However, pre-emption does not occur.

A work-around is to set the number of reducers < available containers.