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From "Xianyin Xin (JIRA)" <j...@apache.org>
Subject [jira] [Resolved] (YARN-3553) TreeSet is not a nice container for organizing schedulableEntities.
Date Mon, 04 May 2015 06:08:06 GMT

     [ https://issues.apache.org/jira/browse/YARN-3553?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel

Xianyin Xin resolved YARN-3553.
    Resolution: Not A Problem

> 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 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.

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