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From "Owen O'Malley (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-2141) speculative execution start up condition based on completion time
Date Thu, 15 Nov 2007 21:53:43 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-2141?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12542895
] 

Owen O'Malley commented on HADOOP-2141:
---------------------------------------

{quote}
There are always going to be such corner cases, and proper speculative execution is the right
solution to such problems.
{quote}

This is exactly backwards. Speculative execution absolutely can *not* be used as a reliability
solution. Applications can and do turn it off. Therefore, the system must be completely reliable
without speculative execution. Furthermore, if there is a "freezing" problem it may well strike
the speculative task also, which would lock the job. Speculative execution is a pure optimization.
It is not for reliability.

> speculative execution start up condition based on completion time
> -----------------------------------------------------------------
>
>                 Key: HADOOP-2141
>                 URL: https://issues.apache.org/jira/browse/HADOOP-2141
>             Project: Hadoop
>          Issue Type: Improvement
>          Components: mapred
>    Affects Versions: 0.15.0
>            Reporter: Koji Noguchi
>            Assignee: Arun C Murthy
>             Fix For: 0.16.0
>
>
> We had one job with speculative execution hang.
> 4 reduce tasks were stuck with 95% completion because of a bad disk. 
> Devaraj pointed out 
> bq . One of the conditions that must be met for launching a speculative instance of a
task is that it must be at least 20% behind the average progress, and this is not true here.
> It would be nice if speculative execution also starts up when tasks stop making progress.
> Devaraj suggested 
> bq. Maybe, we should introduce a condition for average completion time for tasks in the
speculative execution check. 

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