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From "Devaraj Das (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-1270) Randomize the fetch of map outputs
Date Thu, 03 May 2007 18:12:15 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-1270?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12493440

Devaraj Das commented on HADOOP-1270:

Also, this patch solves another issue that was reported as part of HADOOP-1183. That is, it
solves the problem of old map events (corresponding to the failing fetches from locations
which are currently not valid) overwriting new valid map events. This is because the datastructure,
knownOutputs, has been made a List in this patch (earlier it was a Map) and so MapOutputLocations
will get appended to the list rather than being overwritten.

> Randomize the fetch of map outputs
> ----------------------------------
>                 Key: HADOOP-1270
>                 URL: https://issues.apache.org/jira/browse/HADOOP-1270
>             Project: Hadoop
>          Issue Type: Improvement
>          Components: mapred
>            Reporter: Arun C Murthy
>         Assigned To: Arun C Murthy
>             Fix For: 0.13.0
>         Attachments: HADOOP-1270_20070425_1.patch
> HADOOP-248 did away with random probing of maps for locating map outputs and instead
we now rely on TaskCompletionEvents for the same. 
> However we lost out on the benefit that the randomization in probing resulted in an added
benefit where the map's jetty isn't overloaded with requests for the outputs. We have now
a situation where a map completes, the JT is notified, *all* the reduces get the TaskCompletionEvent
and pretty much swamp the poor map's jetty and this repeats for each map.
> I propose we make a minor change where we collect a set of TaskCompletionEvents and randomize
the list before firing the fetches. Should help fix this mass-hysteria at the map's jetty.
> Thoughts?

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