Hadoop job任务分配

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　　mapreduce.cluster.{map|reduce}memory.mb: These options define the default amount of virtual memory that should be allocated for MapReduce tasks running in the cluster. They typically match the default values set for the options mapreduce.{map|reduce}.memory.mb. They help in the calculation of the total amount of virtual memory available for MapReduce tasks on a slave, using the following equation:
　　Total virtual memory for all MapReduce tasks = (mapreduce.cluster.mapmemory.mb * mapreduce.tasktracker.map.tasks.maximum) + (mapreduce.cluster.reducememory.mb * mapreduce.tasktracker.reduce.tasks.maximum)
　　Typically, reduce tasks require more memory than map tasks. Hence a higher value is recommended for mapreduce.cluster.reducememory.mb. The value is specified in MB. To set a value of 2GB for reduce tasks, set mapreduce.cluster.reducememory.mb to 2048.