There are some situations in which the scheduler can mark a task set as a "zombie" before the task set has completed all of its tasks. For example:
(a) When a task fails b/c of a FetchFailed
(b) When a stage completes because two different attempts create all the ShuffleMapOutput, though no attempt has completed all its tasks (at least, this should result in the task set being marked as zombie, see
(there may be others, I'm not sure if this list is exhaustive.)
Marking a taskset as zombie prevents any additional tasks from getting scheduled, however it does not cancel all currently running tasks. We should cancel all running to avoid wasting resources (and also to make the behavior a little more clear to the end user). Rather than canceling tasks in each case piecemeal, we should refactor the scheduler so that these two actions are always taken together – canceling tasks should go hand-in-hand with marking the taskset as zombie.
Some implementation notes:
- We should change taskSetManager.isZombie to be private and put it behind a method like markZombie or something.
- marking a stage as zombie before the all tasks have completed does not necessarily mean the stage attempt has failed. In case (a), the stage attempt has failed, but in stage (b) we are not canceling b/c of a failure, rather just b/c no more tasks are needed.
- taskScheduler.cancelTasks always marks the task set as zombie. However, it also has some side-effects like logging that the stage has failed and creating a TaskSetFailed event, which we don't want eg. in case (b) when nothing has failed. So it may need some additional refactoring to go along w/ markZombie.
- SchedulerBackend's are free to not implement killTask, so we need to be sure to catch the UnsupportedOperationException s
- Testing this might benefit from