[
https://issues.apache.org/jira/browse/HADOOP-2141?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12542861
]
Runping Qi commented on HADOOP-2141:
------------------------------------
The above proposal sounds reasonable.
Here are some points to consider:
1. A speculative execution for a mapper (reducer) is started
only if there are no pending non-speculative mappers (reducers)
2. We should estimate the expected finish time for a mapper(reducer) based on
its
current progression state and progression rate.
A speculative execution for a mapper (reducer) is starte only if the projected
finish time
is far away than the average execution time of mappers(reducers)
3. It is a bit treaky to compute the average execution of reducers.
If a reducer started before the map phase completed, then the overalp period
should \
be taken out.
4. If a reducer is stucked at shuffling state, the real reason for the stall
may be related to the machine(s)
where the needed map outputs sit. Launching a speculative execution of the
reducer may not help.
In this case, we may need to declare the concerned mappers are gone and re-run
them.
> 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.
--
This message is automatically generated by JIRA.
-
You can reply to this email to add a comment to the issue online.
