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From "Matei Zaharia (JIRA)" <j...@apache.org>
Subject [jira] Updated: (HADOOP-4665) Add preemption to the fair scheduler
Date Thu, 08 Jan 2009 06:42:44 GMT

     [ https://issues.apache.org/jira/browse/HADOOP-4665?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel

Matei Zaharia updated HADOOP-4665:

    Attachment: fs-preemption-v0.patch

Here is an initial version of the patch for review. The main thing missing is unit tests.

The patch adds two things. First there's the preemption, which works as described in the issue
- jobs may preempt others if either they aren't receiving their guaranteed share for some
time, or they are at below half their fair share and negative deficit for some time. The times
can be configured in the fair scheduler config file and thus modified at runtime, and the
guaranteed share timeouts are per pool. On top of this, to aid with debugging and development
of the fair scheduler in the future, there is a scheduler event log, which is disabled by
default but creates some event logs in tab-separated format in $hadoop.log.dir/fairscheduler
if you turn it on. These are meant to be nitty-gritty detailed logs with machine-parsable
event types rather than the "human-readable" logs that go into the standard log4j log for
the JobTracker. They are also potentially much larger on a large cluster, which is why they're
off by default.

I'm running this through hudson to see whether there are complaints from findbugs, checkstyle,
etc, but I will include some unit tests in the final patch.

> 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
>         Attachments: fs-preemption-v0.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
> - 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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