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From "Siddharth Seth (Updated) (JIRA)" <j...@apache.org>
Subject [jira] [Updated] (MAPREDUCE-3721) Race in shuffle can cause it to hang
Date Wed, 25 Jan 2012 22:42:41 GMT

     [ https://issues.apache.org/jira/browse/MAPREDUCE-3721?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Siddharth Seth updated MAPREDUCE-3721:
--------------------------------------

    Attachment: MR-3721.txt

Patch adds another variable - commitMemory (Fetch complete size). A merge is triggered only
if this size exceeds mergeThreshold. Added a check to ensure mergeThreshold is greater than
the maxSingleShuffleLimit.
Earlier - usedMemory (reserved) was used for this computation - which meant a single  segment
way below mergeThreshold could lead to a merge to disk.

Have run several gridmix runs with the patch applied - without a hang. Not including a unit
test - writing one would likely change way more in the shuffle code to be able to recreate
the scenario.
                
> Race in shuffle can cause it to hang
> ------------------------------------
>
>                 Key: MAPREDUCE-3721
>                 URL: https://issues.apache.org/jira/browse/MAPREDUCE-3721
>             Project: Hadoop Map/Reduce
>          Issue Type: Bug
>          Components: mrv2
>    Affects Versions: 0.23.0
>            Reporter: Siddharth Seth
>            Assignee: Siddharth Seth
>            Priority: Blocker
>         Attachments: MR-3721.txt
>
>
> If all current {{Fetcher}}s complete while an in-memory merge is in progress - shuffle
could hang. 
> Specifically - if the memory freed by an in-memory merge does not bring {{MergeManager.usedMemory}}
below {{MergeManager.memoryLimit}} and all current Fetchers complete before the in-memory
merge completes, another in-memory merge will not be triggered - and shuffle will hang. (All
new fetchers are asked to WAIT).

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