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RE: guessing number of reducers.

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:[EMAIL PROTECTED]]
Sent: Wednesday, November 21, 2012 11:39 AM
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 occurring...
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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