Xianyin Xin created YARN-3553:
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Summary: TreeSet is not a nice container for organizing
schedulableEntities.
Key: YARN-3553
URL: https://issues.apache.org/jira/browse/YARN-3553
Project: Hadoop YARN
Issue Type: Wish
Components: scheduler
Reporter: Xianyin Xin
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,
{code}
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);
}
{code}
This method try to remove the schedulableEntity first and then insert 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 since the
element of a TreeSet is identified by comparator. Any implement class of this
abstract class should overwrite this method, but few does. AND, if we want to
make any modification of a schedulableEntity, we must remember the order:
remove, modify the attributes(used for comparing), insert.
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.
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