[jira] [Commented] (AURORA-121) Make the preemptor more efficient

["Bill Farner (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/AURORA-121?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14230398#comment-14230398 ] 

Bill Farner commented on AURORA-121:
------------------------------------

https://reviews.apache.org/r/28572/

> Make the preemptor more efficient
> ---------------------------------
>
>                 Key: AURORA-121
>                 URL: https://issues.apache.org/jira/browse/AURORA-121
>             Project: Aurora
>          Issue Type: Story
>          Components: Scheduler
>            Reporter: Bill Farner
>            Assignee: Bill Farner
>              Labels: O3
>
> When {{TaskSchedulerImpl}} fails to find an open slot for a task, it falls back to the preemptor:
> {code}
> if (!offerQueue.launchFirst(getAssignerFunction(taskId, task))) {
>   // Task could not be scheduled.
>   maybePreemptFor(taskId);
>   return TaskSchedulerResult.TRY_AGAIN;
> }
> {code}
> 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:
> {noformat}
> 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)
> {noformat}
> 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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