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    <title>MongoDB Jira</title>
    <link>https://jira.mongodb.org</link>
    <description>This file is an XML representation of an issue</description>
    <language>en-us</language>    <build-info>
        <version>9.7.1</version>
        <build-number>970001</build-number>
        <build-date>13-04-2023</build-date>
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<item>
            <title>[SERVER-27829] Reduce performance impact of DocumentSourceCursor batching</title>
                <link>https://jira.mongodb.org/browse/SERVER-27829</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;On a large collection with index on &quot;orderdate&quot; compare find and equivalent aggregate first batches:&lt;/p&gt;

&lt;p/&gt;
&lt;div id=&quot;syntaxplugin&quot; class=&quot;syntaxplugin&quot; style=&quot;border: 1px dashed #bbb; border-radius: 5px !important; overflow: auto; max-height: 30em;&quot;&gt;
&lt;table cellspacing=&quot;0&quot; cellpadding=&quot;0&quot; border=&quot;0&quot; width=&quot;100%&quot; style=&quot;font-size: 1em; line-height: 1.4em !important; font-weight: normal; font-style: normal; color: black;&quot;&gt;
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						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;  margin-top: 10px;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;command: find { find: &quot;orders&quot;, filter: { orderdate: { $gte: new Date(759024000000) } }, sort: { orderdate: -1.0 }, projection: { _id: 0.0, orderdate: 1.0 } } planSummary: IXSCAN { orderdate: 1 } cursorid:53712819499 keysExamined:101 docsExamined:0 numYields:0 nreturned:101 reslen:2903  0ms&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   margin-bottom: 10px;  width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;command: aggregate { aggregate: &quot;orders&quot;, pipeline: [ { $match: { orderdate: { $gte: new Date(759024000000) } } }, { $sort: { orderdate: -1.0 } }, { $project: { _id: 0.0, orderdate: 1.0 } } ], cursor: {} } planSummary: IXSCAN { orderdate: 1 } cursorid:52250968997 keysExamined:20972 docsExamined:0 numYields:163 nreturned:101 reslen:2903 11ms&lt;/span&gt;&lt;/pre&gt;
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&lt;p/&gt;

&lt;p&gt;The main performance difference seems to come from keysExamined which is 101 in case of find and equal to I&apos;m not sure what in the second case (there are 2406 distinct orderdate values, collection size is 1500000 and this filter reduces it to 1034289 documents).&lt;/p&gt;</description>
                <environment></environment>
        <key id="349556">SERVER-27829</key>
            <summary>Reduce performance impact of DocumentSourceCursor batching</summary>
                <type id="4" iconUrl="https://jira.mongodb.org/secure/viewavatar?size=xsmall&amp;avatarId=14710&amp;avatarType=issuetype">Improvement</type>
                                            <priority id="3" iconUrl="https://jira.mongodb.org/images/icons/priorities/major.svg">Major - P3</priority>
                        <status id="10038" iconUrl="https://jira.mongodb.org/images/icons/subtask.gif" description="">Backlog</status>
                    <statusCategory id="2" key="new" colorName="default"/>
                                    <resolution id="-1">Unresolved</resolution>
                                        <assignee username="backlog-query-execution">Backlog - Query Execution</assignee>
                                    <reporter username="asya.kamsky@mongodb.com">Asya Kamsky</reporter>
                        <labels>
                            <label>QFB</label>
                            <label>performance</label>
                            <label>query-44-grooming</label>
                    </labels>
                <created>Thu, 26 Jan 2017 23:48:23 +0000</created>
                <updated>Tue, 6 Dec 2022 04:07:16 +0000</updated>
                                                                            <component>Aggregation Framework</component>
                                        <votes>0</votes>
                                    <watches>12</watches>
                                                                                                                <comments>
                            <comment id="1487304" author="asya" created="Fri, 27 Jan 2017 21:28:49 +0000"  >&lt;p&gt;It looks at the keys for first 4MBs worth of documents, vs find 101 keys.  Note it caused aggregation to take 11ms vs 0ms for find.&lt;/p&gt;</comment>
                            <comment id="1486800" author="david.storch" created="Fri, 27 Jan 2017 15:24:45 +0000"  >&lt;p&gt;As part of the work to make find and agg share performance properties, we should look into getting rid of batching inside &lt;tt&gt;DocumentSourceCursor&lt;/tt&gt;. I don&apos;t think it would be hard to make pipelines like this fully streaming.&lt;/p&gt;</comment>
                            <comment id="1486725" author="charlie.swanson" created="Fri, 27 Jan 2017 14:17:13 +0000"  >&lt;p&gt;This is because the cursor stage of the aggregation will internally batch results. The current batch size is 4MB, but was recently reduced from 16MB in &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-27406&quot; title=&quot;Reduce DocumentSourceCursor batch size from 16MB to 4MB&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-27406&quot;&gt;&lt;del&gt;SERVER-27406&lt;/del&gt;&lt;/a&gt;, so it&apos;s possible your version is still using 16MB. To confirm this is the problem, you can change the buffer size via a server parameter:&lt;/p&gt;
&lt;p/&gt;
&lt;div id=&quot;syntaxplugin&quot; class=&quot;syntaxplugin&quot; style=&quot;border: 1px dashed #bbb; border-radius: 5px !important; overflow: auto; max-height: 30em;&quot;&gt;
&lt;table cellspacing=&quot;0&quot; cellpadding=&quot;0&quot; border=&quot;0&quot; width=&quot;100%&quot; style=&quot;font-size: 1em; line-height: 1.4em !important; font-weight: normal; font-style: normal; color: black;&quot;&gt;
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				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;  margin-top: 10px;   margin-bottom: 10px;  width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;&amp;gt; db.adminCommand({setParameter: 1, internalDocumentSourceCursorBatchSizeBytes: &amp;lt;&lt;/span&gt;&lt;span style=&quot;color: #006699; font-weight: bold; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;new&lt;/span&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt; batch size&amp;gt;})&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
			&lt;/tbody&gt;
&lt;/table&gt;
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&lt;p/&gt;
&lt;p&gt;Of course this is a tradeoff, we use a large batch size to hide the overhead of dropping and re-acquiring the lock. A query will hold the lock until it has filled up its first batch to return to the user, but an aggregation will only hold a lock during the cursor stage. If we only returned one document at a time from the cursor stage, we would have to re-acquire and drop the lock each time a result is requested from the cursor, which would add a lot of overhead. The internal buffering/batching is meant to mitigate this overhead.&lt;/p&gt;

&lt;p&gt;If you are only interested in time to first 101, you could add a $limit after the $sort, and it should be passed through to the query layer. You wouldn&apos;t be able to ask for more results from that cursor of course.&lt;/p&gt;</comment>
                    </comments>
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                                            <outwardlinks description="related to">
                                        <issuelink>
            <issuekey id="426901">SERVER-31072</issuekey>
        </issuelink>
                            </outwardlinks>
                                                                <inwardlinks description="is related to">
                                        <issuelink>
            <issuekey id="1306172">SERVER-47370</issuekey>
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