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From "Arun C Murthy (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-3517) The last InMemory merge may be missed
Date Mon, 09 Jun 2008 17:21:45 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-3517?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12603622#action_12603622
] 

Arun C Murthy commented on HADOOP-3517:
---------------------------------------

bq. would just calling doInMemMerge after the while (!exitInMemMerge) loop in InMemFSMergeThread
work?
It probably would work, but as with race-conditions it's hard to _prove_ it works. Either
way, this is hacky at best. *smile*

bq.1. The condition in ShuffleRamManager.reserve is not related to condition in ShuffleRamManager.waitForDataToMerge().
So there could be situations (depends on the various thresholds set) when the merge thread
is waiting for thresholds to be met; and at the same time shuffle is stalled waiting for merge
to happen.
This definitely should not happen - the idea is that amount of data in-memory and no. of in-memory
file are the only thresholds. 'dataAvailable' and 'close' are the variables tracking these.

bq. 2. i think the RamManger.reserve(int requestedSize, InputStream in) interface could be
simplied to RamManger.reserve(int requestedSize). There is no real need to close the connection
in reserve method. The connection could be created after the call to reserve in getMapOutput
method.
No. We need to open to connection to 'figure' _requestedSize_ and we absolutely need to close
the connection if we are waiting for requestedSize to be available...

> The last InMemory merge may be missed
> -------------------------------------
>
>                 Key: HADOOP-3517
>                 URL: https://issues.apache.org/jira/browse/HADOOP-3517
>             Project: Hadoop Core
>          Issue Type: Bug
>          Components: mapred
>    Affects Versions: 0.18.0
>            Reporter: Devaraj Das
>            Assignee: Arun C Murthy
>            Priority: Blocker
>             Fix For: 0.18.0
>
>
> This is post HADOOP-3366. The inmem merge thread has the loop:
> {code}
>         while (!exitInMemMerge) {
>             ramManager.waitForDataToMerge();
>             doInMemMerge();
>           }
> {code}
> The fetchOutputs, at the end of copying everything, does the following:
> {code}
>         exitInMemMerge = true; 
>         ramManager.close();
> {code}
> Now if the merge thread is doing a merge (inside the doInMemMerge method) when the exitInMemMerge
is set to true, the loop will break and the last merge of the files that got shuffled recently
will be skipped. ramManager.close(), that internally does a final notify to the merge thread
also won't have any effect in this case.

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