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        <version>9.7.1</version>
        <build-number>970001</build-number>
        <build-date>13-04-2023</build-date>
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<item>
            <title>[SERVER-85393] use monotonic_buffer_resource to reduce allocations in MetricTree merging algorithm</title>
                <link>https://jira.mongodb.org/browse/SERVER-85393</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;As part of PM-1812 and tcmalloc evaluation, we&apos;ve identified allocator &quot;hot spots&quot; in particular those where allocations are short-lived and freed at the end of a block scope in an RAII-compatible way.  C++17 provides the &lt;tt&gt;std::monotonic_buffer_resource&lt;/tt&gt; which seems perfect for those cases.&lt;/p&gt;

&lt;p&gt;we can switch to std::pmr::vector and std::pmr::string inside the functions. We can probably make up a pmr-based BSONObjBuilder as well.&lt;br/&gt;
The standard containers do this cool trick of propagating their allocators to their elements. This is a reason to use emplace_back instead of push_back, because emplace_back can sneakily provide the extra allocator argument to the element being constructed. So you can have a vector of vector of vector of strings and all objects would share the allocator that was given to the toplevel object.&lt;br/&gt;
&lt;a href=&quot;https://quuxplusone.github.io/blog/2023/06/02/not-so-quick-pmr/#allocator-extended-constructors&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://quuxplusone.github.io/blog/2023/06/02/not-so-quick-pmr/#allocator-extended-constructors&lt;/a&gt;&lt;br/&gt;
&lt;a href=&quot;https://www.cppstd17.com/code/pmr/pmr2.cpp.html&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://www.cppstd17.com/code/pmr/pmr2.cpp.html&lt;/a&gt;&lt;/p&gt;



&lt;p&gt; The real power here is from not bothering with global new and delete for all the interior elements.&lt;/p&gt;

&lt;p&gt;so the monotonic buffer resource ends up owning a very small sequence of geometrically increasing memory buffers. So you have to visit new and delete only a few times, regardless of how many internal allocations the memory resource made.&lt;/p&gt;</description>
                <environment></environment>
        <key id="2550339">SERVER-85393</key>
            <summary>use monotonic_buffer_resource to reduce allocations in MetricTree merging algorithm</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="3" iconUrl="https://jira.mongodb.org/images/icons/statuses/inprogress.png" description="This issue is being actively worked on at the moment by the assignee.">In Progress</status>
                    <statusCategory id="4" key="indeterminate" colorName="inprogress"/>
                                    <resolution id="-1">Unresolved</resolution>
                                        <assignee username="blake.oler@mongodb.com">Blake Oler</assignee>
                                    <reporter username="billy.donahue@mongodb.com">Billy Donahue</reporter>
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                <created>Thu, 18 Jan 2024 18:18:25 +0000</created>
                <updated>Mon, 22 Jan 2024 01:34:06 +0000</updated>
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