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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>
    </build-info>


<item>
            <title>[SERVER-61284] $project to exclude an array is expensive and scales poorly as the number of threads increases</title>
                <link>https://jira.mongodb.org/browse/SERVER-61284</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;Repro script attached. It creates a document of this form:&lt;/p&gt;
&lt;p/&gt;
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					&lt;pre style=&quot;font-size: 1em; margin: 0 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;        other: &quot;xxxxxxx&quot;,&lt;/span&gt;&lt;/pre&gt;
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						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
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&lt;p/&gt;
&lt;p&gt;Then runs an aggregation that uses $project in one of two ways:&lt;/p&gt;
&lt;ul&gt;
	&lt;li&gt;exclude the array field using {$project: {array: 0}}, or&lt;/li&gt;
	&lt;li&gt;include only the other field using {$project: {other: 1}}.&lt;/li&gt;
&lt;/ul&gt;


&lt;p&gt;The exclude version is much slower and also scales very poorly as the number of threads increases, even though both are computing the same result. Following table shows time per $project operation for the two cases and for single threaded vs multi-threaded, showing both&lt;/p&gt;
&lt;ul&gt;
	&lt;li&gt;the poor single-threaded performance for the exclusive version, adn&lt;/li&gt;
	&lt;li&gt;the poor scaling; good scaling would have the multi-threaded time per operation approximately the same as the single-threaded case, up to the number of CPUs.&lt;/li&gt;
&lt;/ul&gt;


&lt;p/&gt;
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					&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;                                      nthreads=1    nthreads=40&lt;/span&gt;&lt;/pre&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;   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;    exclusive: {$project: {array: 0}}      18 ms        631 ms&lt;/span&gt;&lt;/pre&gt;
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					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   margin-bottom: 10px;  width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;    inclusive: {$project: {other: 1}}       1 ms          2 ms&lt;/span&gt;&lt;/pre&gt;
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&lt;p/&gt;
&lt;p&gt;FTDC data and PMP (stack trace) profiling taken from a customer incident show that the scaling bottleneck is the allocator, so it appears that the exclusive projection is doing a very large number of memory allocations, which could explain both the lower single-threaded performance and the poor scaling. In the multi-threaded case a typical stack for the exclusion project shows it waiting for access to the allocator central cache in this stack:&lt;/p&gt;
&lt;p/&gt;
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					&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;#0  0x0000561f79a845e6 in base::internal::SpinLockDelay(int volatile*, int, int) ()&lt;/span&gt;&lt;/pre&gt;
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					&lt;pre style=&quot;font-size: 1em; margin: 0 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;#1  0x0000561f79a84413 in SpinLock::SlowLock() ()&lt;/span&gt;&lt;/pre&gt;
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					&lt;pre style=&quot;font-size: 1em; margin: 0 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;#2  0x0000561f79a86495 in tcmalloc::CentralFreeList::RemoveRange(void**, void**, int) ()&lt;/span&gt;&lt;/pre&gt;
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&lt;p/&gt;</description>
                <environment></environment>
        <key id="1919450">SERVER-61284</key>
            <summary>$project to exclude an array is expensive and scales poorly as the number of threads increases</summary>
                <type id="1" iconUrl="https://jira.mongodb.org/secure/viewavatar?size=xsmall&amp;avatarId=14703&amp;avatarType=issuetype">Bug</type>
                                            <priority id="3" iconUrl="https://jira.mongodb.org/images/icons/priorities/major.svg">Major - P3</priority>
                        <status id="6" iconUrl="https://jira.mongodb.org/images/icons/statuses/closed.png" description="The issue is considered finished, the resolution is correct. Issues which are closed can be reopened.">Closed</status>
                    <statusCategory id="3" key="done" colorName="success"/>
                                    <resolution id="13201">Fixed</resolution>
                                        <assignee username="ivan.fefer@mongodb.com">Ivan Fefer</assignee>
                                    <reporter username="bruce.lucas@mongodb.com">Bruce Lucas</reporter>
                        <labels>
                    </labels>
                <created>Fri, 5 Nov 2021 17:54:31 +0000</created>
                <updated>Sun, 29 Oct 2023 21:46:24 +0000</updated>
                            <resolved>Thu, 20 Oct 2022 08:42:18 +0000</resolved>
                                    <version>5.0.2</version>
                    <version>4.2.17</version>
                    <version>4.4.10</version>
                                    <fixVersion>6.2.0-rc0</fixVersion>
                                                        <votes>1</votes>
                                    <watches>27</watches>
                                                                                                                <comments>
                            <comment id="4914837" author="xgen-internal-githook" created="Thu, 20 Oct 2022 08:38:53 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{&apos;name&apos;: &apos;Ivan Fefer&apos;, &apos;email&apos;: &apos;ivan.fefer@mongodb.com&apos;, &apos;username&apos;: &apos;Fefer-Ivan&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-61284&quot; title=&quot;$project to exclude an array is expensive and scales poorly as the number of threads increases&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-61284&quot;&gt;&lt;del&gt;SERVER-61284&lt;/del&gt;&lt;/a&gt;: Support simple projection optimization for simple exclusion projections&lt;br/&gt;
Branch: master&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/commit/9c348fbfafacc93b06d8d4c5521925c97150089f&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/9c348fbfafacc93b06d8d4c5521925c97150089f&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="4872387" author="JIRAUSER1270969" created="Mon, 3 Oct 2022 12:02:59 +0000"  >&lt;p&gt;Projections with exclusions require whole documents:&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/blob/c8d8c12efb6cf1a009dc2e800ca5879b950d54ea/src/mongo/db/query/projection.cpp#L212&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/blob/c8d8c12efb6cf1a009dc2e800ca5879b950d54ea/src/mongo/db/query/projection.cpp#L212&lt;/a&gt;&lt;/p&gt;

&lt;p&gt;So they are not considered simple:&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/blob/c8d8c12efb6cf1a009dc2e800ca5879b950d54ea/src/mongo/db/query/projection.h#L121&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/blob/c8d8c12efb6cf1a009dc2e800ca5879b950d54ea/src/mongo/db/query/projection.h#L121&lt;/a&gt;&lt;/p&gt;

&lt;p&gt;So they are not eligible to all optimizations as stated here:&#160;&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/blob/c8d8c12efb6cf1a009dc2e800ca5879b950d54ea/src/mongo/db/query/planner_analysis.cpp#L423&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/blob/c8d8c12efb6cf1a009dc2e800ca5879b950d54ea/src/mongo/db/query/planner_analysis.cpp#L423&lt;/a&gt;&lt;/p&gt;

&lt;p&gt;But actually ProjectionNodeSimple works with materialized documents, so it is probably fine to move it out of isSimple check.&lt;/p&gt;</comment>
                            <comment id="4872313" author="JIRAUSER1270969" created="Mon, 3 Oct 2022 11:49:29 +0000"  >&lt;p&gt;Looking at implementation of ProjectionStageSimple in Classic engine: &lt;a href=&quot;https://github.com/mongodb/mongo/blob/c8d8c12efb6cf1a009dc2e800ca5879b950d54ea/src/mongo/db/exec/projection.cpp#L282&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/blob/c8d8c12efb6cf1a009dc2e800ca5879b950d54ea/src/mongo/db/exec/projection.cpp#L282&lt;/a&gt;&lt;/p&gt;

&lt;p&gt;It looks like it can be easy to create a solution that supports exclusion of non-dotted fields as well.&lt;/p&gt;</comment>
                            <comment id="4872275" author="JIRAUSER1270969" created="Mon, 3 Oct 2022 11:34:24 +0000"  >&lt;p&gt;Looks like the main reason for the difference is that we have multiple projection implementations: generic one that is slow and faster ones, but they can only be used in specific cases&lt;/p&gt;

&lt;p&gt;&lt;a href=&quot;https://github.com/mongodb/mongo/blob/c8d8c12efb6cf1a009dc2e800ca5879b950d54ea/src/mongo/db/query/query_solution.h#L851&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/blob/c8d8c12efb6cf1a009dc2e800ca5879b950d54ea/src/mongo/db/query/query_solution.h#L851&lt;/a&gt;&lt;/p&gt;

&lt;p&gt;In case of inclusion, we can use this one:&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/blob/c8d8c12efb6cf1a009dc2e800ca5879b950d54ea/src/mongo/db/query/query_solution.h#L954&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/blob/c8d8c12efb6cf1a009dc2e800ca5879b950d54ea/src/mongo/db/query/query_solution.h#L954&lt;/a&gt;&lt;/p&gt;

&lt;p&gt;&amp;gt; fast-path for when the projection only has &lt;b&gt;inclusions on non-dotted fields&lt;/b&gt;&lt;br/&gt;
Because in general we don&apos;t have a schema, we can&apos;t guarantee that we don&apos;t have non-dotted fields, so we can&apos;t use this version of projection in case of {array: 0}.&#160;&lt;/p&gt;

&lt;p&gt;So the fix for this case will be to add this check, using a schema if it is present of some meta-data, possibly.&lt;/p&gt;</comment>
                            <comment id="4872250" author="JIRAUSER1270969" created="Mon, 3 Oct 2022 11:22:43 +0000"  >&lt;p&gt;SBE and Classic both have this issue&lt;/p&gt;</comment>
                            <comment id="4872247" author="JIRAUSER1270969" created="Mon, 3 Oct 2022 11:22:12 +0000"  >&lt;p&gt;&lt;span class=&quot;nobr&quot;&gt;&lt;a href=&quot;https://jira.mongodb.org/secure/attachment/405511/405511_project.js&quot; title=&quot;project.js attached to SERVER-61284&quot;&gt;project.js&lt;sup&gt;&lt;img class=&quot;rendericon&quot; src=&quot;https://jira.mongodb.org/images/icons/link_attachment_7.gif&quot; height=&quot;7&quot; width=&quot;7&quot; align=&quot;absmiddle&quot; alt=&quot;&quot; border=&quot;0&quot;/&gt;&lt;/sup&gt;&lt;/a&gt;&lt;/span&gt;&lt;/p&gt;

&lt;p&gt;Didn&apos;t see reproduce script, made my own.&lt;br/&gt;
Attaching it just in case.&lt;br/&gt;
Problem is reproduced:&#160;&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;
		&lt;tbody &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-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;[js_test:project] starting include_other&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;   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;[js_test:project] include_other: 2ms&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;   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;...&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;   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;[js_test:project] starting exclude_array&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;[js_test:project] exclude_array: 448ms &lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
			&lt;/tbody&gt;
&lt;/table&gt;
&lt;/div&gt;
&lt;p/&gt;</comment>
                            <comment id="4649424" author="JIRAUSER1265607" created="Wed, 29 Jun 2022 23:32:08 +0000"  >&lt;p&gt;Can someone from the eng team to give us a quick ballpark estimation for the fix? &lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=david.storch%40mongodb.com&quot; class=&quot;user-hover&quot; rel=&quot;david.storch@mongodb.com&quot;&gt;david.storch@mongodb.com&lt;/a&gt; &lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=kyle.suarez%40mongodb.com&quot; class=&quot;user-hover&quot; rel=&quot;kyle.suarez@mongodb.com&quot;&gt;kyle.suarez@mongodb.com&lt;/a&gt;&#160;&lt;/p&gt;</comment>
                            <comment id="4521205" author="david.storch" created="Mon, 2 May 2022 21:59:43 +0000"  >&lt;p&gt;This ticket got abandoned, so I&apos;m marking for re-triage. CC &lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=rushan.chen%40mongodb.com&quot; class=&quot;user-hover&quot; rel=&quot;rushan.chen@mongodb.com&quot;&gt;rushan.chen@mongodb.com&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="4172308" author="bruce.lucas@10gen.com" created="Fri, 5 Nov 2021 18:09:19 +0000"  >&lt;p&gt;Thanks for reminding me to check that. I see the same behavior on 5.0.2, 4.4.10, and 4.2.17. The multi-threaded case is actually a little better on 4.2.17, ~400 ms vs ~600 ms.&lt;/p&gt;</comment>
                            <comment id="4172294" author="kyle.suarez" created="Fri, 5 Nov 2021 18:02:50 +0000"  >&lt;p&gt;&lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=bruce.lucas&quot; class=&quot;user-hover&quot; rel=&quot;bruce.lucas&quot;&gt;bruce.lucas&lt;/a&gt;, what version of MongoDB was this tested on?&lt;/p&gt;</comment>
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