[YARN-3553] TreeSet is not a nice container for organizing schedulableEntities.

• Type: Wish
• Status: Resolved
• Priority: Major
• Resolution: Not A Problem
• Component/s: scheduler
• Assignee: Unassigned
• Reporter: Xianyin Xin
• Created: 28/Apr/15 12:39
• Updated: 04/May/15 06:07
• Resolved: 04/May/15 06:07

Description

For TreeSet, element is identified by comparator, not the object reference. If any attributes that used for comparing two elements of an specific element is modified by other methods, the TreeSet will be in an un-sorted state, and cannot become sorted forever except that we reconstruct another TreeSet with the elements. To avoid this, one must be very careful when they try to modify the attributes (such as increase or decrease the used capacity of a schedulabeEntity) of an object.

An example in AbstractComparatorOrderingPolicy.java, Line63,

  protected void reorderSchedulableEntity(S schedulableEntity) {
    //remove, update comparable data, and reinsert to update position in order
    schedulableEntities.remove(schedulableEntity);
    updateSchedulingResourceUsage(
        schedulableEntity.getSchedulingResourceUsage());
    schedulableEntities.add(schedulableEntity);
  }

This method tries to remove the schedulableEntity first and then reinsert it so as to reorder the set. However, the changes of the schedulableEntity should be done in the middle of the above two operations. But the comparator of the class is not clear, so we don't know which attributes of the schedulableEntity was changed. If we changed the schedulableEntity outside the method and then inform the orderingPolicy that we made such a change, the operation "schedulableEntities.remove(schedulableEntity)" would not work correctly since the element of a TreeSet is identified by comparator. Any implement class of this abstract class should overwrite this method, but few does. Another choice is that we make modification of a schedulableEntity manually, but we mustn't forget to reorder the set when we do so and must remember the order: remove, modify the attributes(used for comparing), insert, or use an iterator to mark the schedulableEntity so that we can remove and reinsert it correctly.

YARN-897 is an example that we fell into the trap. If the comparator become complex in future, e.g., we consider other types of resources in comparator, such traps will be more and disperse anywhere, which makes it easy to let a TreeSet become a un-sorted state.