[YARN-3309] Capacity scheduler can wait a very long time for node locality

• Type: Bug
• Status: Open
• Priority: Major
• Resolution: Unresolved
• Affects Version/s: 2.6.0
• Component/s: capacityscheduler
• Assignee: Unassigned
• Reporter: Nathan Roberts
• Created: 09/Mar/15 21:13
• Updated: 10/Mar/15 01:27

Description

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.

Issue Links

• is related to: SLIDER-799 AM to decide when to relax placement policy from specific host to rack/cluster