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From Li Li <fancye...@gmail.com>
Subject Re: how to solve reducer memory problem?
Date Thu, 03 Apr 2014 09:10:58 GMT
*mapred.child.java.opts=-Xmx2g*


On Thu, Apr 3, 2014 at 5:10 PM, Li Li <fancyerii@gmail.com> wrote:

> 2g
>
>
> On Thu, Apr 3, 2014 at 1:30 PM, Stanley Shi <sshi@gopivotal.com> wrote:
>
>> This doesn't seem like related with the data size.
>>
>> How much memory do you use for the reducer?
>>
>> Regards,
>> *Stanley Shi,*
>>
>>
>>
>> On Thu, Apr 3, 2014 at 8:04 AM, Li Li <fancyerii@gmail.com> wrote:
>>
>>> I have a map reduce program that do some matrix operations. in the
>>> reducer, it will average many large matrix(each matrix takes up
>>> 400+MB(said by Map output bytes). so if there 50 matrix to a reducer,
>>> then the total memory usage is 20GB. so the reduce task got exception:
>>>
>>> FATAL org.apache.hadoop.mapred.Child: Error running child :
>>> java.lang.OutOfMemoryError: Java heap space
>>> at org.apache.hadoop.mapred.IFile$Reader.readNextBlock(IFile.java:344)
>>> at org.apache.hadoop.mapred.IFile$Reader.next(IFile.java:406)
>>> at org.apache.hadoop.mapred.Merger$Segment.next(Merger.java:238)
>>> at org.apache.hadoop.mapred.Merger$MergeQueue.merge(Merger.java:438)
>>> at org.apache.hadoop.mapred.Merger.merge(Merger.java:142)
>>> at
>>> org.apache.hadoop.mapred.ReduceTask$ReduceCopier.createKVIterator(ReduceTask.java:2539)
>>> at
>>> org.apache.hadoop.mapred.ReduceTask$ReduceCopier.access$400(ReduceTask.java:661)
>>> at org.apache.hadoop.mapred.ReduceTask.run(ReduceTask.java:399)
>>> at org.apache.hadoop.mapred.Child$4.run(Child.java:255)
>>> at java.security.AccessController.doPrivileged(Native Method)
>>> at javax.security.auth.Subject.doAs(Subject.java:415)
>>> at
>>> org.apache.hadoop.security.UserGroupInformation.doAs(UserGroupInformation.java:1190)
>>> at org.apache.hadoop.mapred.Child.main(Child.java:249)
>>>
>>> one method I can come up with is use Combiner to save sums of some
>>> matrixs and their count
>>> but it still can solve the problem because the combiner is not fully
>>> controled by me.
>>>
>>
>>
>

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