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From "Amareshwari Sri Ramadasu (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-2391) Speculative Execution race condition with output paths
Date Fri, 08 Feb 2008 05:55:07 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-2391?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12566919#action_12566919
] 

Amareshwari Sri Ramadasu commented on HADOOP-2391:
--------------------------------------------------

We can create task directory, job directory and files as suggested by Eric and add check for
existence of parent directory in tasks. And throw an exception if it doesnt exist. This will
take care of creating output files. However we will have issue of tasks creating side files
until we fix  HADOOP-2759.  So, we can fix this for 16.1 and once HADOOP-2759 gets fixed,
creating side files will be fixed automatically.
Thoughts?

> Speculative Execution race condition with output paths
> ------------------------------------------------------
>
>                 Key: HADOOP-2391
>                 URL: https://issues.apache.org/jira/browse/HADOOP-2391
>             Project: Hadoop Core
>          Issue Type: Bug
>          Components: mapred
>         Environment: all
>            Reporter: Dennis Kubes
>            Assignee: Amareshwari Sri Ramadasu
>             Fix For: 0.16.1
>
>         Attachments: HADOOP-2391-1-20071211.patch
>
>
> I am tracking a problem where when speculative execution is enabled, there is a race
condition when trying to read output paths from a previously completed job.  More specifically
when reduce tasks run their output is put into a working directory under the task name until
the task in completed.  The directory name is something like workdir/_taskid.  Upon completion
the output get moved into workdir.  Regular tasks are checked for this move and not considered
completed until this move is made.  I have not verified it but all indications point to speculative
tasks NOT having this same check for completion and more importantly removal when killed.
 So what we end up with when trying to read the output of previous tasks with speculative
execution enabled is the possibility that previous workdir/_taskid will be present when the
output directory is read by a chained job.  Here is an error when supports my theory:
> Generator: org.apache.hadoop.ipc.RemoteException: java.io.IOException: Cannot open filename
/u01/hadoop/mapred/temp/generate-temp-1197104928603/_task_200712080949_0005_r_000014_1
>         at org.apache.hadoop.dfs.NameNode.open(NameNode.java:234)
>         at sun.reflect.GeneratedMethodAccessor64.invoke(Unknown Source)
>         at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
>         at java.lang.reflect.Method.invoke(Method.java:597)
>         at org.apache.hadoop.ipc.RPC$Server.call(RPC.java:389)
>         at org.apache.hadoop.ipc.Server$Handler.run(Server.java:644)
>         at org.apache.hadoop.ipc.Client.call(Client.java:507)
>         at org.apache.hadoop.ipc.RPC$Invoker.invoke(RPC.java:186)
>         at org.apache.hadoop.dfs.$Proxy0.open(Unknown Source)
>         at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
>         at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
>         at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
>         at java.lang.reflect.Method.invoke(Method.java:597)
>         at org.apache.hadoop.io.retry.RetryInvocationHandler.invokeMethod(RetryInvocationHandler.java:82)
>         at org.apache.hadoop.io.retry.RetryInvocationHandler.invoke(RetryInvocationHandler.java:59)
>         at org.apache.hadoop.dfs.$Proxy0.open(Unknown Source)
>         at org.apache.hadoop.dfs.DFSClient$DFSInputStream.openInfo(DFSClient.java:839)
>         at org.apache.hadoop.dfs.DFSClient$DFSInputStream.<init>(DFSClient.java:831)
>         at org.apache.hadoop.dfs.DFSClient.open(DFSClient.java:263)
>         at org.apache.hadoop.dfs.DistributedFileSystem.open(DistributedFileSystem.java:114)
>         at org.apache.hadoop.io.SequenceFile$Reader.<init>(SequenceFile.java:1356)
>         at org.apache.hadoop.io.SequenceFile$Reader.<init>(SequenceFile.java:1349)
>         at org.apache.hadoop.io.SequenceFile$Reader.<init>(SequenceFile.java:1344)
>         at org.apache.hadoop.mapred.SequenceFileOutputFormat.getReaders(SequenceFileOutputFormat.java:87)
>         at org.apache.nutch.crawl.Generator.generate(Generator.java:429)
>         at org.apache.nutch.crawl.Generator.run(Generator.java:563)
>         at org.apache.hadoop.util.ToolRunner.run(ToolRunner.java:65)
>         at org.apache.hadoop.util.ToolBase.doMain(ToolBase.java:54)
>         at org.apache.nutch.crawl.Generator.main(Generator.java:526)
> I will continue to research this and post as I make progress on tracking down this bug.

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