我遇到了一个奇怪的问题。当我在大型数据集（>1TB 压缩文本文件）上运行 Hadoop 作业时，一些 reduce 任务失败，堆栈跟踪如下：
java.io.IOException: Task: attempt_201104061411_0002_r_000044_0 - The reduce copier failed
at org.apache.hadoop.mapred.ReduceTask.run(ReduceTask.java:385)
at org.apache.hadoop.mapred.Child$4.run(Child.java:240)
at java.security.AccessController.doPrivileged(Native Method)
at javax.security.auth.Subject.doAs(Subject.java:396)
at org.apache.hadoop.security.UserGroupInformation.doAs(UserGroupInformation.java:1115)
at org.apache.hadoop.mapred.Child.main(Child.java:234)
Caused by: java.io.IOException: Intermediate merge failed
at org.apache.hadoop.mapred.ReduceTask$ReduceCopier$InMemFSMergeThread.doInMemMerge(ReduceTask.java:2714)
at org.apache.hadoop.mapred.ReduceTask$ReduceCopier$InMemFSMergeThread.run(ReduceTask.java:2639)
Caused by: java.lang.RuntimeException: java.io.EOFException
at org.apache.hadoop.io.WritableComparator.compare(WritableComparator.java:128)
at org.apache.hadoop.mapred.Merger$MergeQueue.lessThan(Merger.java:373)
at org.apache.hadoop.util.PriorityQueue.downHeap(PriorityQueue.java:139)
at org.apache.hadoop.util.PriorityQueue.adjustTop(PriorityQueue.java:103)
at org.apache.hadoop.mapred.Merger$MergeQueue.adjustPriorityQueue(Merger.java:335)
at org.apache.hadoop.mapred.Merger$MergeQueue.next(Merger.java:350)
at org.apache.hadoop.mapred.Merger.writeFile(Merger.java:156)
at org.apache.hadoop.mapred.ReduceTask$ReduceCopier$InMemFSMergeThread.doInMemMerge(ReduceTask.java:2698)
... 1 more
Caused by: java.io.EOFException
at java.io.DataInputStream.readInt(DataInputStream.java:375)
at com.__.hadoop.pixel.segments.IpCookieCountFilter$IpAndIpCookieCount.readFields(IpCookieCountFilter.java:241)
at org.apache.hadoop.io.WritableComparator.compare(WritableComparator.java:125)
... 8 more
java.io.IOException: Task: attempt_201104061411_0002_r_000056_0 - The reduce copier failed
at org.apache.hadoop.mapred.ReduceTask.run(ReduceTask.java:385)
at org.apache.hadoop.mapred.Child$4.run(Child.java:240)
at java.security.AccessController.doPrivileged(Native Method)
at javax.security.auth.Subject.doAs(Subject.java:396)
at org.apache.hadoop.security.UserGroupInformation.doAs(UserGroupInformation.java:1115)
at org.apache.hadoop.mapred.Child.main(Child.java:234)
Caused by: java.io.IOException: Intermediate merge failed
at org.apache.hadoop.mapred.ReduceTask$ReduceCopier$InMemFSMergeThread.doInMemMerge(ReduceTask.java:2714)
at org.apache.hadoop.mapred.ReduceTask$ReduceCopier$InMemFSMergeThread.run(ReduceTask.java:2639)
Caused by: java.lang.RuntimeException: java.io.EOFException
at org.apache.hadoop.io.WritableComparator.compare(WritableComparator.java:128)
at org.apache.hadoop.mapred.Merger$MergeQueue.lessThan(Merger.java:373)
at org.apache.hadoop.util.PriorityQueue.upHeap(PriorityQueue.java:123)
at org.apache.hadoop.util.PriorityQueue.put(PriorityQueue.java:50)
at org.apache.hadoop.mapred.Merger$MergeQueue.merge(Merger.java:447)
at org.apache.hadoop.mapred.Merger$MergeQueue.merge(Merger.java:381)
at org.apache.hadoop.mapred.Merger.merge(Merger.java:107)
at org.apache.hadoop.mapred.Merger.merge(Merger.java:93)
at org.apache.hadoop.mapred.ReduceTask$ReduceCopier$InMemFSMergeThread.doInMemMerge(ReduceTask.java:2689)
... 1 more
Caused by: java.io.EOFException
at java.io.DataInputStream.readFully(DataInputStream.java:180)
at org.apache.hadoop.io.Text.readString(Text.java:402)
at com.__.hadoop.pixel.segments.IpCookieCountFilter$IpAndIpCookieCount.readFields(IpCookieCountFilter.java:240)
at org.apache.hadoop.io.WritableComparator.compare(WritableComparator.java:122)
... 9 more
并非我所有的减速器都失败了。在我看到其他人失败之前，有几个人经常成功。如您所见，堆栈跟踪似乎总是源自IPAndIPCookieCount.readFields()并始终处于内存合并阶段，但并不总是来自readFields.
在运行较小的数据集（大约是大小的 1/30）时，此作业会成功。作业的输出几乎与输入一样多，但每个输出记录都较短。这项工作本质上是二次排序的实现。
我们正在使用 CDH3 Hadoop 发行版。
这是我的自定义WritableComparable实现：
public static class IpAndIpCookieCount implements WritableComparable<IpAndIpCookieCount> {
private String ip;
private int ipCookieCount;
public IpAndIpCookieCount() {
// empty constructor for hadoop
}
public IpAndIpCookieCount(String ip, int ipCookieCount) {
this.ip = ip;
this.ipCookieCount = ipCookieCount;
}
public String getIp() {
return ip;
}
public int getIpCookieCount() {
return ipCookieCount;
}
@Override
public void readFields(DataInput in) throws IOException {
ip = Text.readString(in);
ipCookieCount = in.readInt();
}
@Override
public void write(DataOutput out) throws IOException {
Text.writeString(out, ip);
out.writeInt(ipCookieCount);
}
@Override
public int compareTo(IpAndIpCookieCount other) {
int firstComparison = ip.compareTo(other.getIp());
if (firstComparison == 0) {
int otherIpCookieCount = other.getIpCookieCount();
if (ipCookieCount == otherIpCookieCount) {
return 0;
} else {
return ipCookieCount < otherIpCookieCount ? 1 : -1;
}
} else {
return firstComparison;
}
}
@Override
public boolean equals(Object o) {
if (o instanceof IpAndIpCookieCount) {
IpAndIpCookieCount other = (IpAndIpCookieCount) o;
return ip.equals(other.getIp()) && ipCookieCount == other.getIpCookieCount();
} else {
return false;
}
}
@Override
public int hashCode() {
return ip.hashCode() ^ ipCookieCount;
}
}
readFields方法很简单，看不出这个类有什么问题。此外，我还看到其他人获得了基本相同的堆栈跟踪：
- http://lucene.472066.n3.nabble.com/Reduce-Copier-Failed-td2120228.html
- https://groups.google.com/a/cloudera.org/group/cdh-user/browse_thread/thread/3544da912bf66506
- http://www.listware.net/201010/hadoop-common-user/70382-merging-of-the-local-fs-files-threw-an-exception-javaioioexception-javalangruntimeexception-javaioeofexception.html
- http://mail-archives.apache.org/mod_mbox/hadoop-mapreduce-user/201101.mbox/%3CSNT135-w58DBCAAC6970BB35B50B9AB7FD0@phx.gbl%3E
- http://web.archiveorange.com/archive/v/5nvvZTgeqwCRQ3F9vEzI
似乎没有人真正弄清楚这背后的问题。最后两个似乎表明这可能是一个内存问题（尽管这些堆栈跟踪不是OutOfMemoryExceptions）。就像该链接列表中的倒数第二个帖子一样，我尝试将减速器的数量设置得更高（最多 999 个），但仍然失败。我（还）没有尝试分配更多的内存来减少任务，因为这需要我们重新配置我们的集群。
这是 Hadoop 中的错误吗？还是我做错了什么？
编辑：我的数据按天分区。如果我运行该作业 7 次，每天一次，则所有 7 次都完成。如果我在所有 7 天内运行一项工作，它就会失败。整个 7 天的大型报告将看到与较小报告完全相同的键（总体上），但显然不是以相同的顺序，在相同的 reducer 等。