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From Florin P <florinp...@yahoo.com>
Subject Re: Problems with MR Job running really slowly
Date Sun, 06 Nov 2011 15:11:24 GMT

  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

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