Expert Contributor
Posts: 82
Registered: ‎05-09-2017

One reducer never completes

[ Edited ]

We are seeing some issues with or MR job and one reducer never completes. it keeps running for 8-9 hrs.  we initially tried increasing the reducer memory and increased number of reducers from 40 to 60. 


we noticed that Reduce Output Records are increasing at a slow pace. noting is being written to disk. 


Out of the 60 reducers reducer # 32 is taking longer. the rest of the reducers complete in around 1 min. we suspect its a data skew issue. How can i further troubleshoot this issue ? or has anyone seen this in the past ? 


Posts: 1,886
Kudos: 425
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Registered: ‎07-31-2013

Re: One reducer never completes

Given that the exact same partition always takes higher time, it does
appear to be a data skew (lot of data with the same key, or an improvable
key choice), or an incorrect hash technique for your keys.

Overall if you find your other reducers have processed 0 or a much smaller
number of input groups/records compared to the longer running one, you can
chalk it down to the above.