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https://issues.apache.org/jira/browse/TEZ-3291?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15319263#comment-15319263
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Bikas Saha commented on TEZ-3291:
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Then that would be a bug to fix. Hopefully thats what the patch is doing.
> Optimize splits grouping when locality information is not available
> -------------------------------------------------------------------
>
> Key: TEZ-3291
> URL: https://issues.apache.org/jira/browse/TEZ-3291
> Project: Apache Tez
> Issue Type: Improvement
> Reporter: Rajesh Balamohan
> Priority: Minor
> Attachments: TEZ-3291.WIP.patch
>
>
> There are scenarios where splits might not contain the location details. S3
> is an example, where all splits would have "localhost" for the location
> details. In such cases, curent split computation does not go through the
> rack local and allow-small groups optimizations and ends up creating small
> number of splits. Depending on clusters this can end creating long running
> map jobs.
> Example with hive:
> ==============
> 1. Inventory table in tpc-ds dataset is partitioned and is relatively a small
> table.
> 2. With query-22, hive requests with the original splits count as 52 and
> overall length of splits themselves is around 12061817 bytes.
> {{tez.grouping.min-size}} was set to 16 MB.
> 3. In tez splits grouping, this ends up creating a single split with 52+
> files be processed in the split. In clusters with split locations, this
> would have landed up with multiple splits since {{allowSmallGroups}} would
> have kicked in.
> But in S3, since everything would have "localhost" all splits get added to
> single group. This makes things a lot worse.
> 4. Depending on the dataset and the format, this can be problematic. For
> instance, file open calls and random seeks can be expensive in S3.
> 5. In this case, 52 files have to be opened and processed by single task in
> sequential fashion. Had it been processed by multiple tasks, response time
> would have drastically reduced.
> E.g log details
> {noformat}
> 2016-06-01 13:48:08,353 [INFO] [InputInitializer {Map 2} #0]
> |split.TezMapredSplitsGrouper|: Grouping splits in Tez
> 2016-06-01 13:48:08,353 [INFO] [InputInitializer {Map 2} #0]
> |split.TezMapredSplitsGrouper|: Desired splits: 110 too large. Desired
> splitLength: 109652 Min splitLength: 16777216 New desired splits: 1 Total
> length: 12061817 Original splits: 52
> 2016-06-01 13:48:08,354 [INFO] [InputInitializer {Map 2} #0]
> |split.TezMapredSplitsGrouper|: Desired numSplits: 1 lengthPerGroup: 12061817
> numLocations: 1 numSplitsPerLocation: 52 numSplitsInGroup: 52 totalLength:
> 12061817 numOriginalSplits: 52 . Grouping by length: true count: false
> 2016-06-01 13:48:08,354 [INFO] [InputInitializer {Map 2} #0]
> |split.TezMapredSplitsGrouper|: Number of splits desired: 1 created: 1
> splitsProcessed: 52
> {noformat}
> Alternate options:
> ==================
> 1. Force Hadoop to provide bogus locations for S3. But not sure, if that
> would be accepted anytime soon. Ref: HADOOP-12878
> 2. Set {{tez.grouping.min-size}} to very very low value. But should the end
> user always be doing this on query to query basis?
> 3. When {{(lengthPerGroup < "tez.grouping.min-size")}}, recompute
> desiredNumSplits only when number of distinct locations in the splits is > 1.
> This would force more number of splits to be generated.
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