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除了map函数输出的所有键都相同之外,什么会导致MapReduce作业中只有一个Reducer?MapReduce低效减速器
除了map函数输出的所有键都相同之外,什么会导致MapReduce作业中只有一个Reducer?MapReduce低效减速器
可能的原因:
<property>
<name>mapred.reduce.tasks</name>
<value>1</value>
<description>The default number of reduce tasks per job. Typically set to 99%
of the cluster's reduce capacity, so that if a node fails the reduces can
still be executed in a single wave.
Ignored when mapred.job.tracker is "local".
</description>
</property>