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https://issues.apache.org/jira/browse/HADOOP-2175?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12541386
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Runping Qi commented on HADOOP-2175:
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The criteria for starting a speculative execution should also include
how much/little progress has been made in the past M minutes.
If a task is detected to be stalled, a speculative execution should be
started, no matter how close the original one is to completion.
> Blacklisted hosts may not be able to serve map outputs
> ------------------------------------------------------
>
> Key: HADOOP-2175
> URL: https://issues.apache.org/jira/browse/HADOOP-2175
> Project: Hadoop
> Issue Type: Bug
> Components: mapred
> Reporter: Runping Qi
> Assignee: Arun C Murthy
>
> After a node fails 4 mappers (tasks), it is added to blacklist thus it will
> no longer accept tasks.
> But, it will continue serve the map outputs of any mappers that ran
> successfully there.
> However, the node may not be able serve the map outputs either.
> This will cause the reducers to mark the corresponding map outputs as from
> slow hosts,
> but continue to try to get the map outputs from that node.
> This may lead to waiting forever.
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