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https://issues.apache.org/jira/browse/MAPREDUCE-2538?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Allen Wittenauer updated MAPREDUCE-2538:
----------------------------------------
Fix Version/s: (was: 0.24.0)
Status: Patch Available (was: Open)
> InputSampler.writePartitionFile() may write duplicate keys
> ----------------------------------------------------------
>
> Key: MAPREDUCE-2538
> URL: https://issues.apache.org/jira/browse/MAPREDUCE-2538
> Project: Hadoop Map/Reduce
> Issue Type: Bug
> Affects Versions: 0.20.2
> Environment: EMR.
> Reporter: Michael White
> Assignee: Devaraj K
> Priority: Minor
> Attachments: MAPREDUCE-2538.patch
>
>
> InputSampler.writePartitionFile() outputs the same key multiple times if the
> input samples have enough of a given key to span multiple partitions. There
> is logic in the code that appears to try to avoid this, but seems incorrect:
> for(int i = 1; i < numPartitions; ++i) {
> int k = Math.round(stepSize * i);
> while (last >= k && comparator.compare(samples[last], samples[k]) == 0) {
> ++k;
> }
> writer.append(samples[k], nullValue);
> last = k;
> }
> The while loop condition "last >= k" is always false. The sample comparison
> after the && never occurs.
> It's not entirely clear what the correct fix is. The current behavior is
> arguably correct mathematically, though the while loop could be elided for
> clarity. If bug MAPREDUCE-1987 were fixed, it would be less of a problem
> (for me at least), since that is where the non-uniqueness causes me problems.
> Alternatively, changing the while to:
> "if( last >= 0) {
> while (comparator.compare(samples[last], samples[k]) >= 0)) {"
> or, optimized for skipping over many duplicates (but arguably less clear):
> "if (last >= 0) {
> while (last >= k || comparator.compare(samples[last], samples[k]) >= 0)) {"
> would probably achieve what the original author intended.
> Perhaps the behavior could be selected by a parameter, e.g. "boolean unique".
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