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From "Amar Kamat (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-1984) some reducer stuck at copy phase and progress extremely slowly
Date Tue, 20 Nov 2007 06:18:43 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-1984?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12543790
] 

Amar Kamat commented on HADOOP-1984:
------------------------------------

Submitting a patch which implements the strategy discussed above. The way it works is as follows
:
1. On the first failed attempt the copy is backed off by {{backoff_init}},  currently set
to *4 sec*
2. For subsequent failures the map output copy is backed off by {{backoff_init}} * {{backoff_base}}^{{num_retries-1}}^,
 {{backoff_base}} currently set to *2*.
3. Backoff continues until the total time spent on the copy is less than {{max_backoff}} which
is user configurable using {{mapred.reduce.copy.backoff}}.
     i.e  {{backoff(1) + backoff(2) + .... backoff(max_retries) ~ max_backoff}},  default
{{max_backoff}} is set to *300 (5 min)*
4. After a total of {{max_backoff}} time the job tracker is notified.
5. This cycle continues for every new map on the host since {{num_retries}} is for a particular
map.


> some reducer stuck at copy phase and progress extremely slowly
> --------------------------------------------------------------
>
>                 Key: HADOOP-1984
>                 URL: https://issues.apache.org/jira/browse/HADOOP-1984
>             Project: Hadoop
>          Issue Type: Bug
>          Components: mapred
>    Affects Versions: 0.16.0
>            Reporter: Runping Qi
>            Assignee: Amar Kamat
>            Priority: Critical
>             Fix For: 0.16.0
>
>         Attachments: HADOOP-1984-simple.patch, HADOOP-1984.patch
>
>
> In many cases, some reducers got stuck at copy phase, progressing extremely slowly.
> The entire cluster seems doing nothing. This causes a very bad long tails of otherwise
well tuned map/red jobs.

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