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From Paul Yang <py...@fb.com>
Subject RE: OutOfMemory errors on joining 2 large tables.
Date Wed, 23 Feb 2011 03:23:15 GMT
Have you taken a look at the distribution of your join keys? If there are a couple join keys
that occur much more frequently than others, the reducers handling those keys will have more
load and may be subject to an OOM.

-----Original Message-----
From: Bennie Schut [mailto:bschut@ebuddy.com] 
Sent: Tuesday, February 22, 2011 12:46 AM
To: user@hive.apache.org
Subject: Re: OutOfMemory errors on joining 2 large tables.

I've just set the "hive.exec.reducers.bytes.per.reducer" to as low as 100k which caused this
job to run with 999 reducers. I still have 5 tasks failing with an outofmemory.

We have jvm reuse set to 8 but dropping it to 1 seems to greatly reduce this problem:
set mapred.job.reuse.jvm.num.tasks = 1;

It's still puzzling me how it can run out of memory. It seems like some of the reducers get
an unequally large share of the work.


On 02/18/2011 10:53 AM, Bennie Schut wrote:
> When we try to join two large tables some of the reducers stop with an
> OutOfMemory exception.
>
> Error: java.lang.OutOfMemoryError: Java heap space
> at
> org.apache.hadoop.mapred.ReduceTask$ReduceCopier$MapOutputCopier.shuffleInMemory(ReduceTask.java:1508)
>
> at
> org.apache.hadoop.mapred.ReduceTask$ReduceCopier$MapOutputCopier.getMapOutput(ReduceTask.java:1408)
>
> at
> org.apache.hadoop.mapred.ReduceTask$ReduceCopier$MapOutputCopier.copyOutput(ReduceTask.java:1261)
>
> at
> org.apache.hadoop.mapred.ReduceTask$ReduceCopier$MapOutputCopier.run(ReduceTask.java:1195)
>
>
>
> When looking at garbage collection for these reduce tasks it's
> continually doing garbage collections.
> Like this:
> 2011-02-17T14:36:08.295+0100: 1250.547: [Full GC [PSYoungGen:
> 111055K->53659K(233024K)] [ParOldGen: 698410K->698410K(699072K)]
> 809466K->752070K(932096K) [PSPermGen: 14450K->14450K(21248K)], 0.1496600
> secs] [Times: user=1.08 sys=0.00, real=0.15 secs]
> 2011-02-17T14:36:08.600+0100: 1250.851: [Full GC [PSYoungGen:
> 111057K->53660K(233024K)] [ParOldGen: 698410K->698410K(699072K)]
> 809468K->752070K(932096K) [PSPermGen: 14450K->14450K(21248K)], 0.1360010
> secs] [Times: user=1.00 sys=0.01, real=0.13 secs]
> 2011-02-17T14:36:08.915+0100: 1251.167: [Full GC [PSYoungGen:
> 111058K->53659K(233024K)] [ParOldGen: 698410K->698410K(699072K)]
> 809468K->752070K(932096K) [PSPermGen: 14450K->14450K(21248K)], 0.1325960
> secs] [Times: user=0.94 sys=0.00, real=0.14 secs]
> 2011-02-17T14:36:09.205+0100: 1251.457: [Full GC [PSYoungGen:
> 111055K->53659K(233024K)] [ParOldGen: 698410K->698410K(699072K)]
> 809466K->752070K(932096K) [PSPermGen: 14450K->14450K(21248K)], 0.1301610
> secs] [Times: user=0.99 sys=0.00, real=0.13 secs]
>
>
> "mapred.child.java.opts" set to "-Xmx1024M -XX:+UseCompressedOops
> -XX:+UseParallelOldGC -XX:+UseNUMA -Djava.net.preferIPv4Stack=true
> -verbose:gc -XX:+PrintGCDateStamps -XX:+PrintGCDetails
> -Xloggc:/opt/hadoop/logs/task_@taskid@.gc.log"
>
> I've been reducing this parameter "hive.exec.reducers.bytes.per.reducer"
> to as low as 200M but I still get the OutOfMemory errors. I would have
> expected this would drop the amount of data send to the reducers and
> thus not have the OutOfMemory errors to happen.
>
> Any idea's on why this happens?
>
> I'm using a trunk build from around 2011-02-03


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