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        <build-number>970001</build-number>
        <build-date>13-04-2023</build-date>
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<item>
            <title>[SERVER-43096] Pre-allocate smaller buffer for &quot;distinct&quot; commands</title>
                <link>https://jira.mongodb.org/browse/SERVER-43096</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;We &lt;a href=&quot;https://github.com/mongodb/mongo/blob/af7a4a8a840f1a519045a165f558047d9c8af158/src/mongo/db/commands/distinct.cpp#L233&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;allocate a 16 MB buffer&lt;/a&gt; to hold the results of every &quot;distinct&quot; command, no matter how big or small the result is. This can create a performance problem: every buffer is allocated directly from and returned directly to the tcmalloc pageheap, which is moderately expensive, and is a serial bottleneck.&lt;/p&gt;

&lt;p&gt;Also, tcmalloc decommits memory (returns it to the o/s) at a rate more-or-less proportional to the rate at which memory is returned to the pageheap, so this high rate of memory returned to the pageheap can create a high rate of decommitting and the re-commiting memory, which is a very expensive operation.&lt;/p&gt;

&lt;p&gt;We could consider pre-allocating a much smaller buffer here and allowing it to grow as needed.&lt;/p&gt;</description>
                <environment></environment>
        <key id="912997">SERVER-43096</key>
            <summary>Pre-allocate smaller buffer for &quot;distinct&quot; commands</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="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="james.wahlin@mongodb.com">James Wahlin</assignee>
                                    <reporter username="bruce.lucas@mongodb.com">Bruce Lucas</reporter>
                        <labels>
                    </labels>
                <created>Fri, 30 Aug 2019 14:33:03 +0000</created>
                <updated>Sun, 29 Oct 2023 22:17:34 +0000</updated>
                            <resolved>Tue, 8 Oct 2019 14:00:52 +0000</resolved>
                                                    <fixVersion>4.2.1</fixVersion>
                    <fixVersion>4.3.1</fixVersion>
                                                        <votes>0</votes>
                                    <watches>16</watches>
                                                                                                                <comments>
                            <comment id="2518980" author="xgen-internal-githook" created="Tue, 5 Nov 2019 21:37:01 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{&apos;name&apos;: &apos;James Wahlin&apos;, &apos;username&apos;: &apos;jameswahlin&apos;, &apos;email&apos;: &apos;james.wahlin@mongodb.com&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-43096&quot; title=&quot;Pre-allocate smaller buffer for &amp;quot;distinct&amp;quot; commands&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-43096&quot;&gt;&lt;del&gt;SERVER-43096&lt;/del&gt;&lt;/a&gt; Pre-allocate smaller buffer for &quot;distinct&quot; commands&lt;/p&gt;

&lt;p&gt;(cherry picked from commit 22256673a9db8f9cf44bdb149fe20a599ecb7e9a)&lt;br/&gt;
(cherry picked from commit 1e29423eedf119d055b60aad28c086a0cf5460ac)&lt;br/&gt;
Branch: v4.0&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/commit/b37a2dd6d573e126a07cf97c7c3bc295cfb679e5&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/b37a2dd6d573e126a07cf97c7c3bc295cfb679e5&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="2483705" author="xgen-internal-githook" created="Tue, 15 Oct 2019 20:25:32 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{&apos;name&apos;: &apos;James Wahlin&apos;, &apos;username&apos;: &apos;jameswahlin&apos;, &apos;email&apos;: &apos;james.wahlin@mongodb.com&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-43096&quot; title=&quot;Pre-allocate smaller buffer for &amp;quot;distinct&amp;quot; commands&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-43096&quot;&gt;&lt;del&gt;SERVER-43096&lt;/del&gt;&lt;/a&gt; Pre-allocate smaller buffer for &quot;distinct&quot; commands&lt;/p&gt;

&lt;p&gt;(cherry picked from commit 22256673a9db8f9cf44bdb149fe20a599ecb7e9a)&lt;br/&gt;
Branch: v4.2&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/commit/1e29423eedf119d055b60aad28c086a0cf5460ac&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/1e29423eedf119d055b60aad28c086a0cf5460ac&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="2471476" author="xgen-internal-githook" created="Tue, 8 Oct 2019 13:59:01 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{&apos;username&apos;: &apos;jameswahlin&apos;, &apos;email&apos;: &apos;james.wahlin@mongodb.com&apos;, &apos;name&apos;: &apos;James Wahlin&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-43096&quot; title=&quot;Pre-allocate smaller buffer for &amp;quot;distinct&amp;quot; commands&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-43096&quot;&gt;&lt;del&gt;SERVER-43096&lt;/del&gt;&lt;/a&gt; Pre-allocate smaller buffer for &quot;distinct&quot; commands&lt;br/&gt;
Branch: master&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/commit/22256673a9db8f9cf44bdb149fe20a599ecb7e9a&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/22256673a9db8f9cf44bdb149fe20a599ecb7e9a&lt;/a&gt;&lt;/p&gt;</comment>
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