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https://issues.apache.org/jira/browse/TEZ-1400?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14091978#comment-14091978
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Bikas Saha commented on TEZ-1400:
---------------------------------
Reducer 4 should not start until reducer 3 has started some tasks. Unless
Reducer 4 has min threshold set to 0. IMO, min threshold should not be set to 0
but to some small number such that at least 1 source task is needed to trigger
start.
> Reducers stuck when enabling auto-reduce parallelism (MRR case)
> ---------------------------------------------------------------
>
> Key: TEZ-1400
> URL: https://issues.apache.org/jira/browse/TEZ-1400
> Project: Apache Tez
> Issue Type: Bug
> Affects Versions: 0.5.0
> Reporter: Rajesh Balamohan
> Assignee: Rajesh Balamohan
> Labels: performance
> Attachments: dag.dot
>
>
> In M -> R1 -> R2 case, if R1 is optimized by auto-parallelism R2 gets stuck
> waiting for events.
> e.g
> Map 1: 0/1 Map 2: -/- Map 5: 0/1 Map 6: 0/1 Map 7: 0/1
> Reducer 3: 0/23 Reducer 4: 0/1
> ...
> ...
> Map 1: 1/1 Map 2: 148(+13)/161 Map 5: 1/1 Map 6: 1/1 Map
> 7: 1/1 Reducer 3: 0(+3)/3 Reducer 4: 0(+1)/1 <== Auto reduce
> parallelism kicks in
> ..
> Map 1: 1/1 Map 2: 161/161 Map 5: 1/1 Map 6: 1/1 Map 7: 1/1
> Reducer 3: 3/3 Reducer 4: 0(+1)/1
> Job is stuck waiting for events in Reducer 4.
> [fetcher [Reducer_3] #23]
> org.apache.tez.runtime.library.common.shuffle.impl.ShuffleScheduler: copy(3
> of 23 at 0.02 MB/s) <=== *Waiting for 20 more partitions, even though
> Reducer3 has been optimized to use 3 reducers
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