You have set mapreduce.job.reduces=0 in your job configuration, but your job still runs reduce tasks. What is the likely cause?

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Hadoop - Performance Tuning

You have set mapreduce.job.reduces=0 in your job configuration, but your job still runs reduce tasks. What is the likely cause?

AThe job uses a custom partitioner forcing reduce tasks

BThe parameter is overridden by command line or code

CReduce tasks run by default regardless of setting

DMap tasks are mistakenly counted as reduce tasks

Step-by-Step Solution

Solution:

Step 1: Understand effect of mapreduce.job.reduces=0
This setting means no reduce tasks should run.
Step 2: Identify why reduce tasks still run
If reduce tasks run, likely the setting is overridden by command line options or programmatic code.
Final Answer:
The parameter is overridden by command line or code -> Option B
Quick Check:
Overrides cause unexpected reduce tasks [OK]

Quick Trick: Check for overrides if reduce count ignored [OK]

Common Mistakes:

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