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From Amareshwari Sri Ramadasu <amar...@yahoo-inc.com>
Subject Re: Bugs in 0.16.0?
Date Mon, 03 Mar 2008 04:09:10 GMT
Holden Robbins wrote:
> Hello,
> I'm just starting to dig into Hadoop and testing it's feasibility for large scale development
> I was wondering if anyone else being affected by these issues using hadoop 0.16.0?
> I searched Jira, and I'm not sure if I saw anything that specifically fit some of these:
> 1) The symlinks for the distributed cache in the task directory are being created as
'null' directory links (stated another way, the name of the symbolic link in the directory
is the string literal "null").  Am I doing something wrong to cause this, or do not many people
use this functionality?
If you want create symlinks for distributed cache the url has to have 
symlink field like hdfs://host:port/<absolute-path>#<link>. And 
mapred.create.symlink must be set to "yes".
If mapred.create.symlink is yes and link field is not provided, 
distributed cache will create a symlink with literal "null" as you said.
> 2) I'm running into an issue where the job is giving errors in the form:
> 08/03/01 09:44:25 INFO mapred.JobClient: Task Id : task_200803010908_0001_r_000002_0,
Status : FAILED
> Shuffle Error: Exceeded MAX_FAILED_UNIQUE_FETCHES; bailing-out.
> 08/03/01 09:44:25 WARN mapred.JobClient: Error reading task outputGo-Box4
> 08/03/01 09:44:25 WARN mapred.JobClient: Error reading task outputGo-Box4
> The jobs appear to never finish the reducing once this happens.   The tasks themselves
are long running map tasks (up to 10 minutes per input), as far as I understand from the Jira
posts this  is related to the MAX_FAILED_UNIQUE_FETCHES being hard coded to 4?  Is there a
known work around or fix in the pipeline?
> Possible related jira post: https://issues.apache.org/jira/browse/HADOOP-2220
> Improving the way the shuffling mechanism works may also help? https://issues.apache.org/jira/browse/HADOOP-1339
> I've tried setting:
> <property>
>   <name>mapred.reduce.copy.backoff</name>
>   <value>1440</value>
>   <description>The maximum amount of time (in seconds) a reducer spends on  fetching
one map output before declaring it as failed.</description>
> </property>
>  which should be 24 minutes, with no effect.
> 3) Lastly, it would seem beneficial for jobs that have significant startup overhead and
memory requirements to not be run in separate JVMs for each task.  Along these lines, it looks
like someone submitted a patch for JVM-reuse a while back, but it wasn't commited? https://issues.apache.org/jira/browse/HADOOP-249
> Probably a question for the dev mailing list, but if I wanted to modify hadoop to allow
threading tasks, rather than running independent JVMs, is there any reason someone hasn't
done this yet?  Or am I overlooking something?
> Thanks,
> -Holden

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