[YARN-8436] FSParentQueue: Comparison method violates its general contract

• Type: Bug
• Status: Resolved
• Priority: Minor
• Resolution: Fixed
• Affects Version/s: 3.1.0
• Fix Version/s: 3.2.0
• Component/s: fairscheduler
• Hadoop Flags: Reviewed
• Assignee: Wilfred Spiegelenburg
• Reporter: Wilfred Spiegelenburg
• Created: 19/Jun/18 22:39
• Updated: 26/Feb/20 05:29
• Resolved: 19/Jul/18 20:23

Description

The ResourceManager can fail while sorting queues if an update comes in:

FATAL org.apache.hadoop.yarn.server.resourcemanager.ResourceManager: Error in handling event type NODE_UPDATE to the scheduler
java.lang.IllegalArgumentException: Comparison method violates its general contract!
	at java.util.TimSort.mergeLo(TimSort.java:777)
	at java.util.TimSort.mergeAt(TimSort.java:514)
...
	at java.util.Collections.sort(Collections.java:175)
	at org.apache.hadoop.yarn.server.resourcemanager.scheduler.fair.FSParentQueue.assignContainer(FSParentQueue.java:223)

The reason it breaks is a change in the sorted object itself. 
This is why it fails:

• an update from a node comes in as a heartbeat.
• the update triggers a check to see if we can assign a container on the node.
• walk over the queue hierarchy to find a queue to assign a container to: top down.
• for each parent queue we sort the child queues in assignContainer to decide which queue to descent into.
• we lock the parent queue when sort to prevent changes, but we do not lock the child queues that we are sorting.

If during this sorting a different node update changes a child queue then we allow that. This means that the objects that we are trying to sort now might be out of order. That causes the issue with the comparator. The comparator itself is not broken.

Issue Links

• is related to: YARN-9940 avoid continuous scheduling thread crashes while sorting nodes get 'Comparison method violates its general contract'