[Hadoop]

在运行mapreduce的时候，出现Error: GC overhead limit exceeded，查看log日志，发现异常信息为

2015-12-11 11:48:44,716 FATAL [main] org.apache.hadoop.mapred.YarnChild: Error running child : java.lang.OutOfMemoryError: GC overhead limit exceeded
    at java.io.DataInputStream.readUTF(DataInputStream.java:661)
    at java.io.DataInputStream.readUTF(DataInputStream.java:564)
    at xxxx.readFields(DateDimension.java:186)
    at xxxx.readFields(StatsUserDimension.java:67)
    at xxxx.readFields(StatsBrowserDimension.java:68)
    at org.apache.hadoop.io.WritableComparator.compare(WritableComparator.java:158)
    at org.apache.hadoop.mapreduce.task.ReduceContextImpl.nextKeyValue(ReduceContextImpl.java:158)
    at org.apache.hadoop.mapreduce.task.ReduceContextImpl$ValueIterator.next(ReduceContextImpl.java:239)
    at xxx.reduce(BrowserReducer.java:37)
    at xxx.reduce(BrowserReducer.java:16)
    at org.apache.hadoop.mapreduce.Reducer.run(Reducer.java:171)
    at org.apache.hadoop.mapred.ReduceTask.runNewReducer(ReduceTask.java:627)
    at org.apache.hadoop.mapred.ReduceTask.run(ReduceTask.java:389)
    at org.apache.hadoop.mapred.YarnChild$2.run(YarnChild.java:168)
    at java.security.AccessController.doPrivileged(Native Method)
    at javax.security.auth.Subject.doAs(Subject.java:415)
    at org.apache.hadoop.security.UserGroupInformation.doAs(UserGroupInformation.java:1614)
    at org.apache.hadoop.mapred.YarnChild.main(YarnChild.java:163)

从异常中我们可以看到，在reduce读取一下个数据的时候，出现内存不够的问题，从代码中我发现再reduce端使用了读个map集合，这样会导致内存不够的问题。在hadoop2.x中默认Container的yarn child jvm堆大小为200M，通过参数mapred.child.java.opts指定，可以在job提交的时候给定，是一个客户端生效的参数，配置在mapred-site.xml文件中，通过将该参数修改为-Xms200m -Xmx1000m来更改jvm堆大小，异常解决。

参数名称	默认值	描述
mapred.child.java.opts	-Xmx200m	定义mapreduce执行的container容器的执行jvm参数
mapred.map.child.java.opts		单独指定map阶段的执行jvm参数
mapred.reduce.child.java.opts		单独指定reduce阶段的执行jvm参数
mapreduce.admin.map.child.java.opts	-Djava.net.preferIPv4Stack=true -Dhadoop.metrics.log.level=WARN	管理员指定map阶段执行的jvm参数
mapreduce.admin.reduce.child.java.opts	-Djava.net.preferIPv4Stack=true -Dhadoop.metrics.log.level=WARN	管理员指定reduce阶段的执行jvm参数

 上述五个参数生效的分别执行顺序为：

　　map阶段：mapreduce.admin.map.child.java.opts < mapred.child.java.opts < mapred.map.child.java.opts， 也就是说最终会采用mapred.map.child.java.opts定义的jvm参数，如果有冲突的话。

　　reduce阶段：mapreduce.admin.reduce.child.java.opts < mapred.child.java.opts < mapred.reduce.child.java.opts

 hadoop源码参考：org.apache.hadoop.mapred.MapReduceChildJVM.getChildJavaOpts方法。

private static String getChildJavaOpts(JobConf jobConf, boolean isMapTask) {
    String userClasspath = "";
    String adminClasspath = "";
    if (isMapTask) {
        userClasspath = jobConf.get(JobConf.MAPRED_MAP_TASK_JAVA_OPTS,
                jobConf.get(JobConf.MAPRED_TASK_JAVA_OPTS,
                        JobConf.DEFAULT_MAPRED_TASK_JAVA_OPTS));
        adminClasspath = jobConf.get(
                MRJobConfig.MAPRED_MAP_ADMIN_JAVA_OPTS,
                MRJobConfig.DEFAULT_MAPRED_ADMIN_JAVA_OPTS);
    } else {
        userClasspath = jobConf.get(JobConf.MAPRED_REDUCE_TASK_JAVA_OPTS,
                jobConf.get(JobConf.MAPRED_TASK_JAVA_OPTS,
                        JobConf.DEFAULT_MAPRED_TASK_JAVA_OPTS));
        adminClasspath = jobConf.get(
                MRJobConfig.MAPRED_REDUCE_ADMIN_JAVA_OPTS,
                MRJobConfig.DEFAULT_MAPRED_ADMIN_JAVA_OPTS);
    }

    // Add admin classpath first so it can be overridden by user.
    return adminClasspath + " " + userClasspath;
}