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From "Joydeep Sen Sarma (JIRA)" <j...@apache.org>
Subject [jira] Updated: (MAPREDUCE-2062) speculative execution is too aggressive under certain conditions
Date Sat, 11 Sep 2010 01:27:33 GMT

     [ https://issues.apache.org/jira/browse/MAPREDUCE-2062?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Joydeep Sen Sarma updated MAPREDUCE-2062:
-----------------------------------------

    Environment: hadoop-20 with HADOOP-2141

> speculative execution is too aggressive under certain conditions
> ----------------------------------------------------------------
>
>                 Key: MAPREDUCE-2062
>                 URL: https://issues.apache.org/jira/browse/MAPREDUCE-2062
>             Project: Hadoop Map/Reduce
>          Issue Type: Bug
>          Components: jobtracker
>         Environment: hadoop-20 with HADOOP-2141
>            Reporter: Joydeep Sen Sarma
>
> The function canBeSpeculated has subtle bugs that cause too much speculation in certain
cases.
> - it compares the current progress of the task with the last observed mean of all the
tasks. if only one task is in question - then the progress rate decays as time progresses
(in the absence of updates) and std-dev is zero. So a job with a single reducer or mapper
is almost always speculated.
> - is only a single task has reported progress - then the stddev is zero. so other tasks
may be speculated aggressively.
> - several tasks take a while to report progress initially. they seem to get speculated
as soon as speculative-lag is over. the lag should be configurable at the minimum.

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