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MRUnit单元测试Mapper和Reducer类在内存上独立运行, PipelineMapReduceDriver单线程运行.

LocalJobRunner单线程运行, 且仅有一个 Reducer能够启动

conf.set("mapred.job.tracker", "local"); 
conf.set("fs.default.name", "file:////"); 
FileSystem fs = FileSystem.getLocal(conf);

MiniMRCluster, MiniYarnCluster, MiniDFSCluster 多线程