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From Florin P <florinp...@yahoo.com>
Subject Re: Problems with MR Job running really slowly
Date Mon, 07 Nov 2011 07:58:56 GMT
   The advices that  I gave were based on my experience. They helped me to solve my issues
when I was sending a lot of data to reducers.
  I can give you just two advices,  and hope that the really expert guys from Hadoop and
Cloudera to help you. I'm particular interested in this subject too.
1. Set up the speculative task execution to false (mapred.reduce.tasks.speculative.execution=false)
2. Check your if you have enough space on the HDD (hadoop uses a some temporary folders to
write data - please check the documentation about it)
3. Check if you have memory leaks on your process algorithm. Run the process algorithm in
a separate program and profile it with JProfiler or other tool for profiling. It seems to
me that you have an OutOfMemory Error.

Hope that these helps.

Good luck,

--- On Mon, 11/7/11, Steve Lewis <lordjoe2000@gmail.com> wrote:

From: Steve Lewis <lordjoe2000@gmail.com>
Subject: Re: Problems with MR Job running really slowly
To: mapreduce-user@hadoop.apache.org
Date: Monday, November 7, 2011, 12:13 AM

1)  I am varying both the number of mappers and reducers trying to determine three things 
   a) What are the options I need reducers and mappers to              -  Not have
mappers or reducers killed with GC overhead limit exceeded
                - Minimize execution time for the cluster      I use a custom Splitter and
can adjust the block size for anywhere from 1 Mapper to  hundreds of mappers - 
      For an 8 node cluster I am trying 8,16 and 24 reducers

2) I have been playing with          io,sort,factor - using 100 for now
        io.sort.mb is 400 - use mich higher values and the job will not run

3) I set child,vm.opts to Xmx3000m ( getting simulat results to using 1300

4) My mappreads a single file about 1 GB in size. Each item the splitter delivers (about 1KB)
generates tens of thousands of Key Value pair (<100bytes per value) I can do all the work
of generating the output on one machine (but not the shuffle and sort) in about an hour on
one box but my job is running 
for a many hours without completing. I also got a lot of    
after seeing in other tasksLost task tracker: tracker_glados4.systemsbiology.net:localhost.localdomain/
Caused by: java.lang.NullPointerException
	at org.apache.hadoop.io.compress.BZip2Codec$BZip2CompressionOutputStream.write(BZip2Codec.java:200)
	at org.apache.hadoop.fs.FSDataOutputStream$PositionCache.write(FSDataOutputStream.java:41)
	at java.io.DataOutputStream.writeByte(DataOutputStream.java:136)
	at org.apache.hadoop.io.WritableUtils.writeVLong(WritableUtils.java:263)
	at org.apache.hadoop.io.WritableUtils.writeVInt(WritableUtils.java:243)
	at org.apache.hadoop.mapred.IFile$Writer.close(IFile.java:126)
	at org.apache.hadoop.mapred.MapTask$MapOutputBuffer.sortAndSpill(MapTask.java:1242)
	at org.apache.hadoop.mapred.MapTask$MapOutputBuffer.access$1800(MapTask.java:648)
	at org.apache.hadoop.mapred.MapTask$MapOutputBuffer$SpillThread.run(MapTask.java:1135)

The same job runs well with a smaller data set.  Most of the reason for moving to hadoop
is to allow solutions to scale and I am very concerned at howbadly my larger cases are doing.
The documentation does not say about about tuning parameters for my larger jobs without
running into swap hell or 
GC overhead limit exceeeded.
On Sun, Nov 6, 2011 at 7:11 AM, Florin P <florinpico@yahoo.com> wrote:


  How many reducers you are using?

  Regarding the performance parameters, fist you can increase the size of the io.sort.mb

  It seems that you are sending a lot of amount of data to the reducer. By increasing the
value of this parameter, in the shuffle phase, the framework will not be forced to write/spill
data on the HDD that could be a reason for slowing the process.

  If you are using one reducer, then the whole data is sent over HTTP to that reducer. Another
 thing that you have to think about it.

 Just for a curiosity, try increase also the dfs.block.size to 128 MB. It seems that you
are using the default 64 MB. You'll get less mapper tasks.

  Also, depending what configuration you have on the machine how many cores do you have on
CPU, you can increase the values for

mapred.tasktracker.{map|reduce}.tasks.maximum    The maximum number of Map/Reduce tasks,
which are run simultaneously on a given TaskTracker, individually.      Defaults to 2 (2
maps and 2 reduces), but vary it depending on your hardware

 You can have a look at http://hadoop.apache.org/common/docs/r0.20.2/cluster_setup.html.

  A good book for understanding tuning parameters is Hadoop Definitive Guide by Tom White.

  Hope that the above helps.



--- On Thu, 11/3/11, Steve Lewis <lordjoe2000@gmail.com> wrote:

From: Steve Lewis <lordjoe2000@gmail.com>

Subject: Problems with MR Job running really slowly

To: "mapreduce-user" <mapreduce-user@hadoop.apache.org>

Date: Thursday, November 3, 2011, 11:07 PM

I have a job which takes an xml file - the splitter breaks the file into tags, the mapper
parses each tag and sends the data to the reducer. I am using a custom splitter which reads

 the file looking for start and end tags.

When I run the code in the splitter and the mapper - generating separate tags and parsing
them I can read a file sized at about  500MB containing 12000 tags on my local system in
23 seconds

When I read a file on HDFS on a local cluster I can read and parse the file in 38 seconds

When I run the same code on a eight node cluster I get 7 map tasks. The mappers are taking
190 seconds to handle 100 tags of 

which 200 millisec is parsing and almost all of the rest of the time is in context.write.
A mapper handling 1600 tags takes about 3 hours -These are the statistics for a map task -
it it true that one tag well be sent to about 300 keys but still 3 hours to write 1,5 million
records and 5Gb seems

way excessive 

FileSystemCountersFILE_BYTES_READ 816,935,457HDFS_BYTES_READ 439,554,860FILE_BYTES_WRITTEN

PerformanceTotalScoredScans 1,660

Map-Reduce FrameworkCombine output records0Map input records 6,134Spilled Records 1,690,063Map
output bytes 5,517,423,780Combine input records 0

Map output records 571,475

Anyone want to offer suggestions on how to tune the job better


Steven M. Lewis PhD4221 105th Ave NEKirkland, WA 98033

206-384-1340 (cell)

Skype lordjoe_com

Steven M. Lewis PhD4221 105th Ave NEKirkland, WA 98033206-384-1340 (cell)
Skype lordjoe_com

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