whether you can control tasktracker hadoop number ?

rt. Is already deployed and will be up and running hadoop , I can set in code I need now tasktracker number ? For example, I have 10 machines on a cluster has been at work , I just want to open a mission among five machine work , how to achieve that ? Do not tell me to change the configuration file restart after shutdown . . .
Similarly , datanode whether the number can be controlled ?
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expect answers
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CSDN too shallow ?
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can be set , the system uses a default value that can be achieved through a function call to a class setting .
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only know that this can be set up. Concrete will not ah
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how Meirenhuida ah. .