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From "Kartashov, Andy" <Andy.Kartas...@mpac.ca>
Subject RE: guessing number of reducers.
Date Wed, 21 Nov 2012 16:43:00 GMT

This is what I am using...

After you start your job, visit jobtracker's WebUI <ip-address>:50030
And look for Cluster summary. Reduce Task Capacity shall hint you what optimally set your
number to. I could be wrong but it works for me. :)
Cluster Summary (Heap Size is *** MB/966.69 MB)
Running Map Tasks

Running Reduce Tasks

Total Submissions


Occupied Map Slots

Occupied Reduce Slots

Reserved Map Slots

Reserved Reduce Slots

Map Task Capacity

Reduce Task Capacity

Avg. Tasks/Node

Blacklisted Nodes

Excluded Nodes


From: jamal sasha [mailto:jamalshasha@gmail.com]
Sent: Wednesday, November 21, 2012 11:39 AM
To: user@hadoop.apache.org
Subject: guessing number of reducers.

By default the number of reducers is set to 1..
Is there a good way to guess optimal number of reducers....
Or let's say i have tbs worth of data... mappers are of order 5000 or so...
But ultimately i am calculating , let's say, some average of whole data... say average transaction
Now the output will be just one line in one "part"... rest of them will be empty.So i am guessing
i need loads of reducers but then most of them will be empty but at the same time one reducer
won't suffice..
What's the best way to solve this..
How to guess optimal number of reducers..
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