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  1. Aurora
  2. AURORA-121

Make the preemptor more efficient

    Details

    • Type: Story
    • Status: Resolved
    • Priority: Major
    • Resolution: Fixed
    • Affects Version/s: None
    • Fix Version/s: 0.7.0
    • Component/s: Scheduler
    • Labels:
    • Sprint:
      Twitter Aurora Q4 Sprint 5

      Description

      When TaskSchedulerImpl fails to find an open slot for a task, it falls back to the preemptor:

      if (!offerQueue.launchFirst(getAssignerFunction(taskId, task))) {
        // Task could not be scheduled.
        maybePreemptFor(taskId);
        return TaskSchedulerResult.TRY_AGAIN;
      }
      

      This can be problematic when the task store is large (O(10k tasks)) and there is a steady supply of PENDING tasks not satisfied by open slots. This will manifest as an overall degraded/slow scheduler, and logs of slow queries used for preemption:

      I0125 17:47:36.970 THREAD23 org.apache.aurora.scheduler.storage.mem.MemTaskStore.fetchTasks: Query took 107 ms: TaskQuery(owner:null, environment:null, jobName:null,
      taskIds:null, statuses:[KILLING, ASSIGNED, STARTING, RUNNING, RESTARTING], slaveHost:null, instanceIds:null)
      

      Several approaches come to mind to improve this situation (not mutually exclusive):

      • (easy) More aggressively back off on tasks that cannot be satisfied
      • (easy) Fall back to preemption less frequently
      • (easy) Gather the list of slaves from AttributeStore rather than TaskStore. This breaks the operation up into many smaller queries and reduces the amount of work in cases where a match is found. However, this would actually create more work when a match is not found, so this approach is probably not helpful by itself.
      • (harder) Scan for preemption candidates asynchronously, freeing up the TaskScheduler thread and the storage write lock. Scans could be kicked off by the task scheduler, ideally in a way that doesn't dogpile. This could also be done in a weakly-consistent way to minimally contribute to storage contention.

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            • Assignee:
              wfarner Bill Farner
              Reporter:
              wfarner Bill Farner
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              • Created:
                Updated:
                Resolved: