Hadoop开发遇到的问题之reduce卡住

时间：2023-03-09 20:34:59
Hadoop开发遇到的问题之reduce卡住

遇到的问题描述：在hadoop上面执行程序，程序运行之后能够正常执行。一切似乎都是正常的，然而过了一段时间之后程序便开始阻塞直到程序超时退出（如下）。

14/08/19 21:17:51 INFO mapred.JobClient: map 99% reduce 71%

14/08/19 21:17:54 INFO mapred.JobClient: map 99% reduce 75%

14/08/19 21:17:57 INFO mapred.JobClient: map 99% reduce 79%

14/08/19 21:18:00 INFO mapred.JobClient: map 99% reduce 83%

14/08/19 21:18:03 INFO mapred.JobClient: map 99% reduce 87%

14/08/19 21:18:06 INFO mapred.JobClient: map 99% reduce 91%

出现这个问题是因为程序出现了一些异常，导致task执行失败，然而hadoop并不退出也不重启task。

异常一：程序玻本身的错误

attempt_201408192045_0002_m_000196_2: [2014-08-19 21:16:44 WARN ] [main] (org.apache.hadoop.mapred.Child:291) - Error running child

attempt_201408192045_0002_m_000196_2: java.io.IOException: Index: 0, Size: 0

attempt_201408192045_0002_m_000196_2:   at com.ict.hadoop.WXExtraction$Map.map(WXExtraction.java:61)

attempt_201408192045_0002_m_000196_2:   at com.ict.hadoop.WXExtraction$Map.map(WXExtraction.java:1)

attempt_201408192045_0002_m_000196_2:   at org.apache.hadoop.mapred.MapRunner.run(MapRunner.java:50)

attempt_201408192045_0002_m_000196_2:   at org.apache.hadoop.mapred.MapTask.runOldMapper(MapTask.java:391)

attempt_201408192045_0002_m_000196_2:   at org.apache.hadoop.mapred.MapTask.run(MapTask.java:325)

attempt_201408192045_0002_m_000196_2:   at org.apache.hadoop.mapred.Child$4.run(Child.java:270)

attempt_201408192045_0002_m_000196_2:   at java.security.AccessController.doPrivileged(Native Method)

attempt_201408192045_0002_m_000196_2:   at javax.security.auth.Subject.doAs(Subject.java:416)

attempt_201408192045_0002_m_000196_2:   at org.apache.hadoop.security.UserGroupInformation.doAs(UserGroupInformation.java:1127)

attempt_201408192045_0002_m_000196_2:   at org.apache.hadoop.mapred.Child.main(Child.java:264)

attempt_201408192045_0002_m_000196_2: Caused by: java.lang.IndexOutOfBoundsException: Index: 0, Size: 0

attempt_201408192045_0002_m_000196_2:   at java.util.ArrayList.rangeCheck(ArrayList.java:571)

attempt_201408192045_0002_m_000196_2:   at java.util.ArrayList.get(ArrayList.java:349)

attempt_201408192045_0002_m_000196_2:   at com.ict.wxparser.parser.WXParser.getMsgContent(WXParser.java:188)

attempt_201408192045_0002_m_000196_2:   at com.ict.wxparser.parser.WXParser.parseLine(WXParser.java:137)

attempt_201408192045_0002_m_000196_2:   at com.ict.hadoop.WXExtraction$Map.map(WXExtraction.java:57)

attempt_201408192045_0002_m_000196_2:   ... 9 more

attempt_201408192045_0002_m_000196_2: [2014-08-19 21:16:44 INFO ] [main] (org.apache.hadoop.mapred.Task:956) - Runnning cleanup for the task

14/08/19 21:17:18 INFO mapred.JobClient: Task Id : attempt_201408192045_0002_m_000196_3, Status : FAILED

java.io.IOException: Index: 0, Size: 0

        at com.ict.hadoop.WXExtraction$Map.map(WXExtraction.java:61)

        at com.ict.hadoop.WXExtraction$Map.map(WXExtraction.java:1)

        at org.apache.hadoop.mapred.MapRunner.run(MapRunner.java:50)

        at org.apache.hadoop.mapred.MapTask.runOldMapper(MapTask.java:391)

        at org.apache.hadoop.mapred.MapTask.run(MapTask.java:325)

        at org.apache.hadoop.mapred.Child$4.run(Child.java:270)

        at java.security.AccessController.doPrivileged(Native Method)

        at javax.security.auth.Subject.doAs(Subject.java:416)

        at org.apache.hadoop.security.UserGroupInformation.doAs(UserGroupInformation.java:1127)

        at org.apache.hadoop.mapred.Child.main(Child.java:264)

Caused by: java.lang.IndexOutOfBoundsException: Index: 0, Size: 0

        at java.util.ArrayList.rangeCheck(ArrayList.java:571)

        at java.util.ArrayList.get(ArrayList.java:349)

        at com.ict.wxparser.parser.WXParser.getMsgContent(WXParser.java:188)

        at com.ict.wxparser.parser.WXParser.parseLine(WXParser.java:137)

        at com.ict.hadoop.WXExtraction$Map.map(WXExtrac

解决这个问题的关键在于修改代码使得程序任务能够正常执行。

异常二：org.apache.hadoop.mapred.Child: Error running child : java.lang.OutOfMemoryError:   unable to create new native thread

这个问题说明程序的内存已经溢出，这时候会抛出溢出异常，并导致程序执行失败。

解决方法：

1. 增大hadoop-env.sh 中HADOOP_HEAPSIZE的值

2 .增大 mapred-site.xml 中mapred.child.java.opts的值（默认为200M）

<property>

<name>mapred.child.java.opts</name>

<value>-Xmx2048m</value>

</property>

3. 减小 mapred-site.xml中mapred.tasktracker.map.tasks.maximumde和mapred.tasktracker.reduce.tasks.maximum的值

<property>

<name>mapred.tasktracker.map.tasks.maximum</name>

<value>15</value>

</property>

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