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From "Kartashov, Andy" <Andy.Kartas...@mpac.ca>
Subject RE: mapred.tasktracker.map.tasks.maximum
Date Tue, 13 Nov 2012 15:26:47 GMT

The way I understand it...

# of mappers is calculated by deviding your input by file split size (64Mb default).

So, say your input is 64Gb in size so you will end up with 1000 mappers.

Slots are a different story. It is number of map tasks processed by a single node in parrallel.
It is normally calculated by the number of cores that node is running on (one per core).


From: Mark Kerzner [mailto:mark.kerzner@shmsoft.com]
Sent: Tuesday, November 13, 2012 10:20 AM
To: user@hadoop.apache.org
Subject: Re: mapred.tasktracker.map.tasks.maximum

On Tue, Nov 13, 2012 at 10:18 AM, Serge Blazhiyevskyy <Serge.Blazhiyevskyy@nice.com<mailto:Serge.Blazhiyevskyy@nice.com>>
What hadoop version are we talking about?

From: Mark Kerzner <mark.kerzner@shmsoft.com<mailto:mark.kerzner@shmsoft.com><mailto:mark.kerzner@shmsoft.com<mailto:mark.kerzner@shmsoft.com>>>
Reply-To: "user@hadoop.apache.org<mailto:user@hadoop.apache.org><mailto:user@hadoop.apache.org<mailto:user@hadoop.apache.org>>"
Date: Tuesday, November 13, 2012 5:16 PM
To: Hadoop User <user@hadoop.apache.org<mailto:user@hadoop.apache.org><mailto:user@hadoop.apache.org<mailto:user@hadoop.apache.org>>>
Subject: mapred.tasktracker.map.tasks.maximum


I have a cluster with 4 nodes and 32 many cores on each. My default value for the maximum
number of mappers per slot is 1:

    <!-- see other kb entry about this one. -->
(which I think is wrong).

Howeve, when sizable jobs run, I see 65 mappers working, so it seems that it does create more
than 1 mapper per node.

Questions: what maximum number of mappers would be appropriate in this situation? Is that
the right way to set them?

Thank you. Sincerely,

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