Question

I saw these in a template configuration file:

<property>
    <name>mapred.map.tasks</name>
    <value>2</value>
    <description>The default number of map tasks per job.  Typically set
    to a prime several times greater than number of available hosts.
    Ignored when mapred.job.tracker is "local".  
    </description>
</property>

...

<property>
    <name>mapred.reduce.tasks</name>
    <value>1</value>
    <description>The default number of reduce tasks per job.  Typically set
    to a prime close to the number of available hosts.  Ignored when
    mapred.job.tracker is "local".
    </description>
</property>

I couldn’t find any other reference, neither online nor in the Hadoop O’Reilly book, as to why these should be prime. Anyone have any ideas?

Thanks.

Answer 1

See HADOOP-5519; this is no longer in the configuration file as there was no (or little) reason for it.

I haven’t seen it for at least two versions, and JIRA says it was resolved a couple of years ago.