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From "BELUGA BEHR (JIRA)" <j...@apache.org>
Subject [jira] [Updated] (MAPREDUCE-7057) MergeThread Review
Date Tue, 20 Feb 2018 17:57:00 GMT

     [ https://issues.apache.org/jira/browse/MAPREDUCE-7057?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

BELUGA BEHR updated MAPREDUCE-7057:
-----------------------------------
    Status: Patch Available  (was: Open)

> MergeThread Review
> ------------------
>
>                 Key: MAPREDUCE-7057
>                 URL: https://issues.apache.org/jira/browse/MAPREDUCE-7057
>             Project: Hadoop Map/Reduce
>          Issue Type: Improvement
>          Components: mrv2
>    Affects Versions: 3.0.0
>            Reporter: BELUGA BEHR
>            Priority: Minor
>         Attachments: MAPREDUCE-7057.1.patch
>
>
> Source:
>  [MergeThread.java|https://github.com/apache/hadoop/blob/178751ed8c9d47038acf8616c226f1f52e884feb/hadoop-mapreduce-project/hadoop-mapreduce-client/hadoop-mapreduce-client-core/src/main/java/org/apache/hadoop/mapreduce/task/reduce/MergeThread.java]
> Update this class to use Java 1.8 concurrent package.  There also some corner-cases not
being addressed with the current implementation:
> {code:java|title=MergeThread.java}
> // There is a scenario here where N threads have submitted inputs and are all waiting
for the 'pendingToBeMerged' object.  At this point, imagine the 'close' method is called.
 The close method will run, see nothing in the queue, interrupt the processing thread, and
cause it to exit.  Afterwards, the 'startMerge' threads will all be triggered and add the
inputs to a queue for which there is no consumer.  At this point, the T items have been removed
from the inputs with no way to recover them.  In practice, this may not ever be the case,
but it can be tightened up.
>   public void startMerge(Set<T> inputs) {
>     if (!closed) {
>       numPending.incrementAndGet();
>       List<T> toMergeInputs = new ArrayList<T>();
>       Iterator<T> iter=inputs.iterator();
>       for (int ctr = 0; iter.hasNext() && ctr < mergeFactor; ++ctr) {
>         toMergeInputs.add(iter.next());
>         iter.remove();
>       }
>       LOG.info(getName() + ": Starting merge with " + toMergeInputs.size() + 
>                " segments, while ignoring " + inputs.size() + " segments");
>       synchronized(pendingToBeMerged) {
>         pendingToBeMerged.addLast(toMergeInputs);
>         pendingToBeMerged.notifyAll();
>       }
>     }
>   }
>   public synchronized void close() throws InterruptedException {
>     closed = true;
>     waitForMerge();
>     interrupt();
>   }
> {code}



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