Nathan Roberts created YARN-3309:

             Summary: Capacity scheduler can wait a very long time for node 
                 Key: YARN-3309
             Project: Hadoop YARN
          Issue Type: Bug
          Components: capacityscheduler
    Affects Versions: 2.6.0
            Reporter: Nathan Roberts

The capacity scheduler will delay scheduling a container on a rack-local node 
in hopes that a node-local opportunity will come along (YARN-80). It does this 
by counting the number of missed scheduling opportunities the application has 
had. When the count reaches a certain threshold, the app will accept the 
rack-local node. The documented recommendation is to set this threshold to the 
#nodes in the cluster.

However, there are some early-out optimizations that can lead to this delay 
being a very long time. 
Example in allocateContainersToNode():
   // Try to schedule more if there are no reservations to fulfill
    if (node.getReservedContainer() == null) {
      if (calculator.computeAvailableContainers(node.getAvailableResource(),
        minimumAllocation) > 0) {
        if (LOG.isDebugEnabled()) {
          LOG.debug("Trying to schedule on node: " + node.getNodeName() +
              ", available: " + node.getAvailableResource());
        root.assignContainers(clusterResource, node, false);

So, in a large cluster that is completely full (AvailableResource on each node 
is 0), SchedulingOpportunities will only increase at the rate of container 
completion rate, not the heartbeat rate, which I think was the original 
assumption of YARN-80. On a large cluster, this can lead to an hour+ of skipped 
scheduling opportunities meaning the fifo'ness of a queue is ignored for a very 
long time.

Maybe there should be a time-based limit on this delay as well as a count of  
missed-scheduling opportunities.

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