执行操作：

hadoop jar share/hadoop/mapreduce/hadoop-mapreduce-examples-3.1.0.jar wordcount  /user/today/input  /user/today/output

报错：

错误: 找不到或无法加载主类 org.apache.hadoop.mapreduce.v2.app.MRAppMaster

解决：

在mapred-site.xml 和 yarn-site.xml添加如下

<property>

    <name>yarn.application.classpath</name>

    <value>

        /opt/hadoop-2.6.0/etc/hadoop,

        /opt/hadoop-2.6.0/share/hadoop/common/*,

        /opt/hadoop-2.6.0/share/hadoop/common/lib/*,

        /opt/hadoop-2.6.0/share/hadoop/hdfs/*,

        /opt/hadoop-2.6.0/share/hadoop/hdfs/lib/*,

        /opt/hadoop-2.6.0/share/hadoop/mapreduce/*,

        /opt/hadoop-2.6.0/share/hadoop/mapreduce/lib/*,

        /opt/hadoop-2.6.0/share/hadoop/yarn/*,

        /opt/hadoop-2.6.0/share/hadoop/yarn/lib/*

    </value>

  </property>

报错：

INFO mapreduce.Job: Task Id : attempt_1533612124021_0001_m_000000_2, Status : FAILED

Container [……] is running 482732544B beyond the 'VIRTUAL' memory limit. Current usage: 34.4 MB of 1 GB physical memory used; 2.5 GB of 2.1 GB virtual memory used. Killing container.

解决：

在etc/hadoop/yarn-site.xml文件中，修改检查虚拟内存的属性为false

<property>

    <name>yarn.nodemanager.vmem-check-enabled</name>

    <value>false</value>

</property>

执行成功

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