Siddharth Seth commented on HIVE-17256:

Mostly looks good. Would be nice to have a few more tests on the ordering of 
the various  queues / or even better the scheduler making correct decisions.

A guaranteed task will also replace a non-guaranteed task, irrespective of 
finishable state? Wasn't there some potential for deadlocks with this?

In terms of the todo wtf - iirc fixing that requires making some biggish 
changes in tez internals to prevent the same finish being registered multiple 
times over. Would be good to leave that as a comment if it does not exist, 
instead of the wtf. The original jira where that was added should have more 

> add a notion of a guaranteed task to LLAP
> -----------------------------------------
>                 Key: HIVE-17256
>                 URL: https://issues.apache.org/jira/browse/HIVE-17256
>             Project: Hive
>          Issue Type: Bug
>            Reporter: Sergey Shelukhin
>            Assignee: Sergey Shelukhin
>         Attachments: HIVE-17256.patch
> Tasks are basically on two levels, guaranteed and speculative, with 
> speculative being the default. As long as noone uses the new flag, the tasks 
> behave the same.
> All the tasks that do have the flag also behave the same with regard to each 
> other.
> The difference is that a guaranteed task is always higher priority, and 
> preempts, a speculative task. 

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