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https://issues.apache.org/jira/browse/DRILL-6115?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16375188#comment-16375188
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ASF GitHub Bot commented on DRILL-6115:
---------------------------------------
Github user asfgit closed the pull request at:
https://github.com/apache/drill/pull/1110
> SingleMergeExchange is not scaling up when many minor fragments are allocated
> for a query.
> ------------------------------------------------------------------------------------------
>
> Key: DRILL-6115
> URL: https://issues.apache.org/jira/browse/DRILL-6115
> Project: Apache Drill
> Issue Type: Improvement
> Components: Execution - Relational Operators
> Affects Versions: 1.12.0
> Reporter: Hanumath Rao Maduri
> Assignee: Hanumath Rao Maduri
> Priority: Major
> Labels: ready-to-commit
> Fix For: 1.13.0
>
> Attachments: Enhancing Drill to multiplex ordered merge exchanges.docx
>
>
> SingleMergeExchange is created when a global order is required in the output.
> The following query produces the SingleMergeExchange.
> {code:java}
> 0: jdbc:drill:zk=local> explain plan for select L_LINENUMBER from
> dfs.`/drill/tables/lineitem` order by L_LINENUMBER;
> +------+------+
> | text | json |
> +------+------+
> | 00-00 Screen
> 00-01 Project(L_LINENUMBER=[$0])
> 00-02 SingleMergeExchange(sort0=[0])
> 01-01 SelectionVectorRemover
> 01-02 Sort(sort0=[$0], dir0=[ASC])
> 01-03 HashToRandomExchange(dist0=[[$0]])
> 02-01 Scan(table=[[dfs, /drill/tables/lineitem]],
> groupscan=[JsonTableGroupScan [ScanSpec=JsonScanSpec
> [tableName=maprfs:///drill/tables/lineitem, condition=null],
> columns=[`L_LINENUMBER`], maxwidth=15]])
> {code}
> On a 10 node cluster if the table is huge then DRILL can spawn many minor
> fragments which are all merged on a single node with one merge receiver.
> Doing so will create lot of memory pressure on the receiver node and also
> execution bottleneck. To address this issue, merge receiver should be
> multiphase merge receiver.
> Ideally for large cluster one can introduce tree merges so that merging can
> be done parallel. But as a first step I think it is better to use the
> existing infrastructure for multiplexing operators to generate an OrderedMux
> so that all the minor fragments pertaining to one DRILLBIT should be merged
> and the merged data can be sent across to the receiver operator.
> On a 10 node cluster if each node processes 14 minor fragments.
> Current version of code merges 140 minor fragments
> the proposed version has two level merges 1 - 14 merge in each drillbit which
> is parallel
> and 10 minorfragments are merged at the receiver node.
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