option "-D mapred.tasktracker.map.tasks.maximum=1" does not work when no of
mappers is bigger than no of nodes - always spawns 2 mapers/node
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Key: MAPREDUCE-1781
URL: https://issues.apache.org/jira/browse/MAPREDUCE-1781
Project: Hadoop Map/Reduce
Issue Type: Bug
Components: contrib/streaming
Affects Versions: 0.20.2
Environment: Debian Lenny x64, and Hadoop 0.20.2, 2GB RAM
Reporter: Tudor Vlad
Hello
I am a new user of Hadoop and I have some trouble using Hadoop Streaming and
the "-D mapred.tasktracker.map.tasks.maximum" option.
I'm experimenting with an unmanaged application (C++) which I want to run over
several nodes in 2 scenarios
1) the number of maps (input splits) is equal to the number of nodes
2) the number of maps is a multiple of the number of nodes (5, 10, 20, ...
Initially, when running the tests in scenario 1 I would sometimes get 2
process/node on half the nodes. However I fixed this by adding the optin "-D
mapred.tasktracker.map.tasks.maximum=1", so everything works fine.
In the case of scenario 2 (more maps than nodes) this directive no longer
works, always obtaining 2 processes/node. I tested the even with putting
maximum=5 and I still get 2 processes/node.
The entire command I use is:
/usr/bin/time --format="-duration:\t%e |\t-MFaults:\t%F |\t-ContxtSwitch:\t%w" \
/opt/hadoop/bin/hadoop jar
/opt/hadoop/contrib/streaming/hadoop-0.20.2-streaming.jar \
-D mapred.tasktracker.map.tasks.maximum=1 \
-D mapred.map.tasks=30 \
-D mapred.reduce.tasks=0 \
-D io.file.buffer.size=5242880 \
-libjars "/opt/hadoop/contrib/streaming/hadoop-7debug.jar" \
-input input/test \
-output out1 \
-mapper "/opt/jobdata/script_1k" \
-inputformat "me.MyInputFormat"
Why is this happening and how can I make it work properly (i.e. be able to
limit exactly how many mappers I can have at 1 time per node)?
Thank you in advance
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