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https://issues.apache.org/jira/browse/HADOOP-2980?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12577767#action_12577767
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Devaraj Das commented on HADOOP-2980:
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The '50' can be explained since the default probe size (for
mapcompletionevents) is 50. The probe size increases with the number of copier
threads via the calculation: _max(numCopiers*5, 50)_ So one thing that you
should do is to increase the number of copier threads
(mapred.reduce.parallel.copies to something like 20).
But I am not able to clearly explain the 1 minute interval between the queries.
This is one scenario when it can happen - discussed in HADOOP-1719
(https://issues.apache.org/jira/browse/HADOOP-1719?focusedCommentId=12522954#action_12522954).
The other scenario where this can happen is when there is a failure to fetch
output from a host and the task backs off between 1 & 5 minutes from that host.
But you say that you don't see any exceptions...
BTW the above issues are fixed in 0.16.
> slow reduce copies - map output locations not being fetched even when map
> complete
> ----------------------------------------------------------------------------------
>
> Key: HADOOP-2980
> URL: https://issues.apache.org/jira/browse/HADOOP-2980
> Project: Hadoop Core
> Issue Type: Bug
> Affects Versions: 0.15.3
> Reporter: Joydeep Sen Sarma
>
> maps are long finished. reduces are stuck looking for map locations. they
> make progress - but slowly. it almost seems like they get new map locations
> every minute or so:
> 2008-03-07 18:50:52,737 INFO org.apache.hadoop.mapred.ReduceTask:
> task_200803041231_3586_r_000021_0 done copying
> task_200803041231_3586_m_004620_0 output from hadoop082.sf2p.facebook.com..
> 2008-03-07 18:50:53,733 INFO org.apache.hadoop.mapred.ReduceTask:
> task_200803041231_3586_r_000021_0: Got 0 new map-outputs & 0 obsolete
> map-outputs from tasktracker and 0 map-outputs from previous failures
> 2008-03-07 18:50:53,733 INFO org.apache.hadoop.mapred.ReduceTask:
> task_200803041231_3586_r_000021_0 Got 0 known map output location(s);
> scheduling...
> ...
> 2008-03-07 18:51:49,767 INFO org.apache.hadoop.mapred.ReduceTask:
> task_200803041231_3586_r_000021_0 Got 50 known map output location(s);
> scheduling...
> 2008-03-07 18:51:49,767 INFO org.apache.hadoop.mapred.ReduceTask:
> task_200803041231_3586_r_000021_0 Scheduled 41 of 50 known outputs (0 slow
> hosts and 9 dup hosts)
> they get about 50 locations at a time and this 1 minute delay pattern is
> surprisingly common ..
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