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From James Hammerton <james.hammer...@mendeley.com>
Subject Re: Mappers crashing due to running out of heap space during initialisation
Date Wed, 27 Apr 2011 13:56:27 GMT
Thanks Joey.

I guess the puzzle then is why some of my mappers used up over 312mb,
leaving insufficient room out of the 512mb total we allocate when the job is
no more complex than other jobs that run happily in that space. The memory
usage is independent of the size of my data set, and even the largest items
are only 201k in size and it only ever has a few of them in memory at once.

James

On Wed, Apr 27, 2011 at 12:47 PM, Joey Echeverria <joey@cloudera.com> wrote:

> It was initializing a 200MB buffer to do the sorting of the output in.
> How much space did you allocate the task JVMs (mapred.child.java.opts
> in mapred-site.xml)?
>
> If you didn't change the default, it's set to 200MB which is why you
> would run out of error trying to allocate a 200MB buffer.
>
> -Joey
>
> On Wed, Apr 27, 2011 at 6:02 AM, James Hammerton
> <james.hammerton@mendeley.com> wrote:
> > Hi,
> >
> > I lowered the io.sort.mb to 100mb from 200mb and that allowed my job to
> get
> > through the mapping phase, thanks Chris.
> >
> > However what I don't understand is why the memory got used up in the
> first
> > place when the mapper only buffers the previous input and the maximum
> > serialised size of the objects it's dealing with is 201k.
> >
> > This is why I asked about what Hadoop is doing in the area of code where
> the
> > exception was occurring - as far as I can tell, my mapper code wasn't
> even
> > getting run.
> >
> > Regards,
> >
> > James
> >
> > On Tue, Apr 26, 2011 at 8:02 PM, Chris Douglas <cdouglas@apache.org>
> wrote:
> >>
> >> Lower io.sort.mb or raise the heap size for the task. -C
> >>
> >> On Tue, Apr 26, 2011 at 10:55 AM, James Hammerton
> >> <james.hammerton@mendeley.com> wrote:
> >> > Hi,
> >> >
> >> > I have a job that runs fine with a small data set in
> pseudo-distributed
> >> > mode
> >> > on my desktop workstation but when I run it on our Hadoop cluster it
> >> > falls
> >> > over, crashing during the initialisation of some of the mappers. The
> >> > errors
> >> > look like this:
> >> >
> >> > 2011-04-26 14:34:04,494 FATAL org.apache.hadoop.mapred.TaskTracker:
> >> > Error
> >> > running child : java.lang.OutOfMemoryError: Java heap space
> >> >       at
> >> >
> >> >
> org.apache.hadoop.mapred.MapTask$MapOutputBuffer.<init>(MapTask.java:743)
> >> >
> >> >       at
> >> >
> >> >
> org.apache.hadoop.mapred.MapTask$NewOutputCollector.<init>(MapTask.java:487)
> >> >       at
> org.apache.hadoop.mapred.MapTask.runNewMapper(MapTask.java:575)
> >> >       at org.apache.hadoop.mapred.MapTask.run(MapTask.java:305)
> >> >
> >> >       at org.apache.hadoop.mapred.Child.main(Child.java:170)
> >> >
> >> > The mapper itself buffers only the previous input and the objects are
> >> > small
> >> > (max 201K in size, most under 50k), so I don't know why this is
> >> > happening.
> >> >
> >> > What exactly is happening in the area of code referred to in the stack
> >> > trace?
> >> >
> >> > Cheers,
> >> >
> >> > James
> >> >
> >> > --
> >> > James Hammerton | Senior Data Mining Engineer
> >> > www.mendeley.com/profiles/james-hammerton
> >> >
> >> > Mendeley Limited | London, UK | www.mendeley.com
> >> > Registered in England and Wales | Company Number 6419015
> >> >
> >> >
> >> >
> >> >
> >
> >
> >
> > --
> > James Hammerton | Senior Data Mining Engineer
> > www.mendeley.com/profiles/james-hammerton
> >
> > Mendeley Limited | London, UK | www.mendeley.com
> > Registered in England and Wales | Company Number 6419015
> >
> >
> >
> >
>
>
>
> --
> Joseph Echeverria
> Cloudera, Inc.
> 443.305.9434
>



-- 
James Hammerton | Senior Data Mining Engineer
www.mendeley.com/profiles/james-hammerton

Mendeley Limited | London, UK | www.mendeley.com
Registered in England and Wales | Company Number 6419015

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