Unfortunately this will copy the old HashSet if it exists. Instead, do something like this:

```
if (!executorToTasksActive.contains(eid)) {
  executorToTasksActive(eid) = HashSet[Long]()
}
executorToTasksActive(eid) += taskStart.taskInfo.taskId
```

The `+=` operator will modify the HashSet in place instead of copying it the way `+` would.