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From Harsh J <qwertyman...@gmail.com>
Subject Re: Awareness of Map tasks
Date Fri, 01 Apr 2011 08:20:08 GMT
Hey Matthew,

You can gain some more knowledge on this by reading up on how the
MapReduce parts interact with their DFS counterparts in Hadoop's

Yahoo's resources carry a good graphical representation and
description, for starters:

On Wed, Mar 30, 2011 at 11:51 AM, Matthew John
<tmatthewjohn1988@gmail.com> wrote:
> Hi all,
> Had some queries on Map task's awareness. From what I understand,
> every map task instance is destined to process the data in a specific
> Input split (can be across HDFS blocks).
> 1) Do these map tasks have a unique instance number?

Yes, all tasks carry a unique ID.

> If yes, are they
> mapped to its specific input splits and the mapping is done using what
> parameters (say for eg. map task number to input file byte offset ?) ?
> where exactly is this hash-map preserved (at what level - jobtracker,
> tasktracker or each tasks) ?

Roughly speaking, one TIP object is generated per InputSplit and a
list of this is kept by the JIP object in the memory of the
JobTracker. The scheduler is then responsible for choosing the right
tasktracker for each of the to-be-run TIPs.

> 2) coming to a practical scenario, when I run hadoop in local mode. I
> run a mapreduce job with 10 maps. Since there is an inherent jvm
> parallelism (say the node can afford to run 2 map task jvms
> simultaneously) I assume that there are some map tasks that run
> concurrently. Since HDFS doesnot play a role in this case, how is the
> map task instance - to - input split mapping mechanism carried out ?
> Or do we have a concept of input split at all (will all the maps start
> scanning from the start of the input file) ?

In case of 'local' mode, splits are still generated using simple seek
offsets using a default or supplied split size. Every mapper task then
seeks to its assigned split's start and begins the processing till
split's end is reached.

P.s. If you're delving into code of a current release, give
http://wiki.apache.org/hadoop/HadoopMapRedClasses a read. Pretty
helpful before you dive.

Harsh J

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