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From "Amareshwari Sri Ramadasu (JIRA)" <j...@apache.org>
Subject [jira] Issue Comment Edited: (HADOOP-2391) Speculative Execution race condition with output paths
Date Tue, 05 Feb 2008 09:57:07 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-2391?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12565669#action_12565669
] 

amareshwari edited comment on HADOOP-2391 at 2/5/08 1:55 AM:
--------------------------------------------------------------------------

bq. After more looking I think I have a better understanding of what is happening. Speculative
execution would cause more tasks to be killed because in parallel tasks one task will finish
before the other and the slower task will be killed. Killed tasks are returned as completed
immediately and have their output removed by a separate thread than the thread which completes
a job. Meaning it is possible for a job to complete and return before output for killed tasks
is fully removed.

Yes.  One probable solution is we can store the temporary output files in {noformat} <output_dir>/_temporary/_<taskid>
{noformat}  When the job is complete, we can delete the directory {noformat} <output_dir>/_temporary/
{noformat} . With the same functionality of task commit thread, it gets an exception in discarding
the task output for the killed tasks which is ignored, because it is already deleted. 
Thoughts?

      was (Author: amareshwari):
    bq. After more looking I think I have a better understanding of what is happening. Speculative
execution would cause more tasks to be killed because in parallel tasks one task will finish
before the other and the slower task will be killed. Killed tasks are returned as completed
immediately and have their output removed by a separate thread than the thread which completes
a job. Meaning it is possible for a job to complete and return before output for killed tasks
is fully removed.

Yes.  One probable solution is we can store the temporary output files in <output_dir>/_temporary/_<taskid>.
When the job is complete, we can delete the directory <output_dir>/_temporary/. With
the same functionality of task commit thread, it gets an exception in discarding the task
output for the killed tasks which is ignored, because it is already deleted. 
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: Devaraj Das
>             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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