Lei A good explanation of this can be found on the Hadoop The Definitive Guide by Tom White.
Here is an excerpt that explains a bit the behavior at the reduce side and some possible tweaks to control it. https://www.inkling.com/read/hadoop-definitive-guide-tom-white-3rd/chapter-6/shuffle-and-sort From: [email protected] [mailto:[email protected]] Sent: Tuesday, April 15, 2014 9:29 AM To: user; th Subject: Re: Re: memoryjava.lang.OutOfMemoryError related with number of reducer? Thanks Thomas. Anohter question. I have no idea what is "Failed to merge in memory". Does the 'merge' is the shuffle phase in reducer side? Why it is in memory? Except the two methods(increase reducer number and increase heap size), is there any other alternatives to fix this issue? Thanks a lot. _____ [email protected] From: Thomas Bentsen <mailto:[email protected]> Date: 2014-04-15 21:53 To: user <mailto:[email protected]> Subject: Re: memoryjava.lang.OutOfMemoryError related with number of reducer? When you increase the number of reducers they each have less to work with provided the data is distributed evenly between them - in this case about one third of the original work. It is eessentially the same thing as increasing the heap size - it's just distributed between more reducers. /th On Tue, 2014-04-15 at 20:41 +0800, [email protected] wrote: > I can fix this by changing heap size. > But what confuse me is that when i change the reducer number from 24 > to 84, there's no this error. > > > Any insight on this? > > > Thanks > Lei > Failed to merge in memoryjava.lang.OutOfMemoryError: Java heap space > at java.util.Arrays.copyOf(Arrays.java:2786) > at java.io.ByteArrayOutputStream.write(ByteArrayOutputStream.java:94) > at java.io.DataOutputStream.write(DataOutputStream.java:90) > at java.io.DataOutputStream.writeUTF(DataOutputStream.java:384) > at java.io.DataOutputStream.writeUTF(DataOutputStream.java:306) > at org.apache.pig.data.utils.SedesHelper.writeChararray(SedesHelper.java:66) > at org.apache.pig.data.BinInterSedes.writeDatum(BinInterSedes.java:543) > at org.apache.pig.data.BinInterSedes.writeDatum(BinInterSedes.java:435) > at > org.apache.pig.data.utils.SedesHelper.writeGenericTuple(SedesHelper.java:135) > at org.apache.pig.data.BinInterSedes.writeTuple(BinInterSedes.java:613) > at org.apache.pig.data.BinInterSedes.writeBag(BinInterSedes.java:604) > at org.apache.pig.data.BinInterSedes.writeDatum(BinInterSedes.java:447) > at org.apache.pig.data.BinInterSedes.writeDatum(BinInterSedes.java:435) > at > org.apache.pig.data.utils.SedesHelper.writeGenericTuple(SedesHelper.java:135) > at org.apache.pig.data.BinInterSedes.writeTuple(BinInterSedes.java:613) > at org.apache.pig.data.BinInterSedes.writeDatum(BinInterSedes.java:443) > at org.apache.pig.data.BinInterSedes.writeDatum(BinInterSedes.java:435) > at > org.apache.pig.data.utils.SedesHelper.writeGenericTuple(SedesHelper.java:135) > at org.apache.pig.data.BinInterSedes.writeTuple(BinInterSedes.java:613) > at org.apache.pig.data.BinInterSedes.writeDatum(BinInterSedes.java:443) > at org.apache.pig.data.BinSedesTuple.write(BinSedesTuple.java:41) > at > org.apache.pig.impl.io.PigNullableWritable.write(PigNullableWritable.java:123) > at > org.apache.hadoop.io.serializer.WritableSerialization$WritableSerializer.serialize(WritableSerialization.java:100) > at > org.apache.hadoop.io.serializer.WritableSerialization$WritableSerializer.serialize(WritableSerialization.java:84) > at org.apache.hadoop.mapred.IFile$Writer.append(IFile.java:188) > at > org.apache.hadoop.mapred.Task$CombineOutputCollector.collect(Task.java:1145) > at > org.apache.hadoop.mapred.Task$NewCombinerRunner$OutputConverter.write(Task.java:1456) > at > org.apache.hadoop.mapreduce.task.TaskInputOutputContextImpl.write(TaskInputOutputContextImpl.java:85) > at > org.apache.hadoop.mapreduce.lib.reduce.WrappedReducer$Context.write(WrappedReducer.java:99) > at > org.apache.pig.backend.hadoop.executionengine.mapReduceLayer.PigCombiner$Combine.processOnePackageOutput(PigCombiner.java:201) > at > org.apache.pig.backend.hadoop.executionengine.mapReduceLayer.PigCombiner$Combine.reduce(PigCombiner.java:163) > at > org.apache.pig.backend.hadoop.executionengine.mapReduceLayer.PigCombiner$Combine.reduce(PigCombiner.java:51) > > ______________________________________________________________________ > [email protected]
