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https://issues.apache.org/jira/browse/HADOOP-4665?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12699047#action_12699047
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Vinod K V commented on HADOOP-4665:
-----------------------------------
Agree with Hemanth on having two knobs - one for completely disabling
preemption and one for enabling a dry run.
Regarding leaving some heartbeats before killing tasks, I agree with your
arguments and am fine with leaving that for now as the timeouts may indeed turn
out to be in the order of minutes like you are saying. If it becomes a need
later, we can do it in future in another JIRA issue.
I'm +1 to the rest of the arguments. So I'll be fine with the patch once the
rest of the code comments are incorporated along with a knob for disabling
preemption altogether.
> Add preemption to the fair scheduler
> ------------------------------------
>
> Key: HADOOP-4665
> URL: https://issues.apache.org/jira/browse/HADOOP-4665
> Project: Hadoop Core
> Issue Type: New Feature
> Components: contrib/fair-share
> Reporter: Matei Zaharia
> Assignee: Matei Zaharia
> Fix For: 0.21.0
>
> Attachments: fs-preemption-v0.patch, hadoop-4665-v1.patch,
> hadoop-4665-v1b.patch, hadoop-4665-v2.patch, hadoop-4665-v3.patch,
> hadoop-4665-v4.patch
>
>
> Task preemption is necessary in a multi-user Hadoop cluster for two reasons:
> users might submit long-running tasks by mistake (e.g. an infinite loop in a
> map program), or tasks may be long due to having to process large amounts of
> data. The Fair Scheduler (HADOOP-3746) has a concept of guaranteed capacity
> for certain queues, as well as a goal of providing good performance for
> interactive jobs on average through fair sharing. Therefore, it will support
> preempting under two conditions:
> 1) A job isn't getting its _guaranteed_ share of the cluster for at least T1
> seconds.
> 2) A job is getting significantly less than its _fair_ share for T2 seconds
> (e.g. less than half its share).
> T1 will be chosen smaller than T2 (and will be configurable per queue) to
> meet guarantees quickly. T2 is meant as a last resort in case non-critical
> jobs in queues with no guaranteed capacity are being starved.
> When deciding which tasks to kill to make room for the job, we will use the
> following heuristics:
> - Look for tasks to kill only in jobs that have more than their fair share,
> ordering these by deficit (most overscheduled jobs first).
> - For maps: kill tasks that have run for the least amount of time (limiting
> wasted time).
> - For reduces: similar to maps, but give extra preference for reduces in the
> copy phase where there is not much map output per task (at Facebook, we have
> observed this to be the main time we need preemption - when a job has a long
> map phase and its reducers are mostly sitting idle and filling up slots).
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