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From Joep Rottinghuis <jrottingh...@gmail.com>
Subject Re: Fair Scheduler of Hadoop
Date Mon, 21 Jan 2013 18:00:03 GMT
You could configure it like that if you wanted. Keep in mind that would waste some resources.
Imagine a 10 minute task that has been running for 9 minutes. If you have that task killed
immediately then it would have to be the-scheduled and re-do all 10 minutes.
Give it another minute and the task is complete and out if the way.

So, consider how busy your cluster is overall and how much you are willing to wait for fairness
trading this off against a certain amount of waste.

Cheers,

Joep

Sent from my iPhone

On Jan 21, 2013, at 9:30 AM, Lin Ma <linlma@gmail.com> wrote:

> Hi Joep,
> 
> Excellent answer! I think you have answered my confusions. And one remaining issue after
reading this document again, even it is old. :-)
> 
> It is mentioned, "which will allow you to set how long each pool will wait before preempting
other jobs’ tasks to reach its guaranteed capacity", my question is why each pool need wait
here? If a pool cannot get its guaranteed capacity because of jobs in other pools over use
the capacity, we should kill such jobs immediately? Appreciate if you could elaborate a bit
more why we need wait to get even guaranteed capacity.
> 
> regards,
> Lin
> 
> On Mon, Jan 21, 2013 at 8:24 AM, Joep Rottinghuis <jrottinghuis@gmail.com> wrote:
>> Lin,
>> 
>> The article you are reading us old.
>> Fair scheduler does have preemption.
>> Tasks get killed and rerun later, potentially on a different node.
>> 
>> You can set a minimum / guaranteed capacity. The sum of those across pools would
typically equal the total capacity of your cluster or less.
>> Then you can configure each pool to go beyond that capacity. That would happen if
the cluster is temporary not used to the full capacity.
>> Then when the demand for capacity increases, and jobs are queued in other pools that
are not running at their minimum guaranteed capacity, some long running tasks from jobs in
the pool that is using more than its minimum capacity get killed (to be run later again).
>> 
>> Does that make sense?
>> 
>> Cheers,
>> 
>> Joep
>> 
>> Sent from my iPhone
>> 
>> On Jan 20, 2013, at 6:25 AM, Lin Ma <linlma@gmail.com> wrote:
>> 
>>> Hi guys,
>>> 
>>> I have a quick question regarding to fire scheduler of Hadoop, I am reading this
article => http://blog.cloudera.com/blog/2008/11/job-scheduling-in-hadoop/, my question
is from the following statements, "There is currently no support for preemption of long tasks,
but this is being added in HADOOP-4665, which will allow you to set how long each pool will
wait before preempting other jobs’ tasks to reach its guaranteed capacity.".
>>> 
>>> My questions are,
>>> 
>>> 1. What means "preemption of long tasks"? Kill long running tasks, or pause long
running tasks to give resources to other tasks, or it means something else?
>>> 2. I am also confused about "set how long each pool will wait before preempting
other jobs’ tasks to reach its guaranteed capacity"., what means "reach its guaranteed capacity"?
I think when using fair scheduler, each pool has predefined resources allocation settings
(and the settings guarantees each pool has resources as configured), is that true? In what
situations each pool will not have its guaranteed (or configured) capacity?
>>> 
>>> regards,
>>> Lin
> 

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