FAQ
Reducers fail with OutOfMemoryError while copying Map outputs
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Key: HADOOP-6357
URL: https://issues.apache.org/jira/browse/HADOOP-6357
Project: Hadoop Common
Issue Type: Bug
Affects Versions: 0.20.0
Reporter: Chandra Prakash Bhagtani


Reducers fail while copying Map outputs with following exception

java.lang.OutOfMemoryError: Java heap space at org.apache.hadoop.mapred.ReduceTask$ReduceCopier$MapOutputCopier.shuffleInMemory(ReduceTask.java:1539) at org.apache.hadoop.mapred.ReduceTask$ReduceCopier$MapOutputCopier.getMapOutput(ReduceTask.java:1432) at org.apache.hadoop.mapred.ReduceTask$ReduceCopier$MapOutputCopier.copyOutput(ReduceTask.java:1285) at org.apache.hadoop.mapred.ReduceTask$ReduceCopier$MapOutputCopier.run(ReduceTask.java:1216) ,Error:

Reducer's memory usage keeps on increasing and ultimately exceeds -Xmx value
I even tried with -Xmx6.5g to each reducer but it's still failing

While looking into the reducer logs, I found that reducers were doing shuffleInMemory every time, rather than doing shuffleOnDisk

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