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
web页面 ip：8088
