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From Ravi Phulari <rphul...@yahoo-inc.com>
Subject Re: posted again: how are the splits for map tasks computed?
Date Thu, 25 Mar 2010 04:32:39 GMT
Hello Abhishek ,

Unless you have modified conf/mapred-site.xml file MapReduce will  use configuration values
specified in $HADOOP_HOME/src/mapred/mapred-default.xml
In this configuration file mapred.map.tasks is configured as 2. And due to this your job is
running 2 map tasks.

<property>
  <name>mapred.map.tasks</name>
  <value>2</value>
  <description>The default number of map tasks per job.
  Ignored when mapred.job.tracker is "local".
  </description>
</property>

Hope this helps.
-
Ravi

On 3/24/10 7:27 PM, "abhishek sharma" <absharma@usc.edu> wrote:

I realized that I made a mistake in my earlier post. So here is the correct one.

I have a job ("loadgen") with only 1 input (say) part-00000 of size
1368654 bytes.

So when I submit this job, I get the following output:

INFO mapred.FileInputFormat: Total input paths to process : 1

However, in the JobTracker log, I see the following entry:

 Split info for job:job_201003131110_0043 with 2 splits

and subsequently 2 map tasks are started to process these two splits.
The size of input splits to these 2 map tasks is 6843283. So the input
is divided equally into two splits.

My question is: Why are two map tasks created instead of one and why
is the combined size of the two splits greater than the size of my
input?

I also noticed that if I run the same job with 2 inputs (say)
part-00000 and part-00001, then only 2 map tasks are created.

To my knowledge, the number of map tasks should be the same as the
number of inputs.

Thanks,


Ravi
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