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    <title>MongoDB Jira</title>
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    <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-39295] Use readOnce: true for $sample cursors</title>
                <link>https://jira.mongodb.org/browse/SERVER-39295</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;This is a feature request to enhance the behaviour of the&#160;&lt;tt&gt;$sample&lt;/tt&gt;&#160;aggregation command by adding to the &lt;em&gt;plan optimizer&lt;/em&gt;&#160;the WiredTiger &#8220;readOnce: true&#8221; option for MongoDB cursors (&lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-36068&quot; title=&quot;Expose a user-accessible cursor option to avoid caching data from reads&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-36068&quot;&gt;&lt;del&gt;SERVER-36068&lt;/del&gt;&lt;/a&gt;).&lt;/p&gt;

&lt;p&gt;The intended purpose behind this enhancement is so &lt;tt&gt;$sample&lt;/tt&gt; does not (or is less likely to) cache the result set.&#160; A sample by definition is unlikely to be used again by subsequent samples, thereby caching has no benefit and only serves to add unwanted cache pressure and workload contention.&#160;&lt;/p&gt;</description>
                <environment></environment>
        <key id="680669">SERVER-39295</key>
            <summary>Use readOnce: true for $sample cursors</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="10033" iconUrl="https://jira.mongodb.org/images/icons/statuses/information.png" description="Status for tickets that need to be escalated and unblocked on our team.">Blocked</status>
                    <statusCategory id="4" key="indeterminate" colorName="inprogress"/>
                                    <resolution id="-1">Unresolved</resolution>
                                        <assignee username="backlog-query-execution">Backlog - Query Execution</assignee>
                                    <reporter username="luke.prochazka@mongodb.com">Luke Prochazka</reporter>
                        <labels>
                            <label>qexec-team</label>
                    </labels>
                <created>Thu, 31 Jan 2019 01:21:07 +0000</created>
                <updated>Tue, 6 Dec 2022 03:06:55 +0000</updated>
                                            <version>4.1.7</version>
                                                    <component>Aggregation Framework</component>
                                        <votes>1</votes>
                                    <watches>17</watches>
                                                                                                                <comments>
                            <comment id="2998426" author="ruoxin.xu" created="Wed, 25 Mar 2020 17:49:35 +0000"  >&lt;p&gt;Back to open. Requires performance investigation for &lt;tt&gt;readOnce&lt;/tt&gt; cursors (see &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-47055&quot; title=&quot;Investigate performance regression in initial sync collection scan with readOnce cursors&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-47055&quot;&gt;&lt;del&gt;SERVER-47055&lt;/del&gt;&lt;/a&gt; / &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-47118&quot; title=&quot;Investigate the behavior of read_once cursors&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-47118&quot;&gt;SERVER-47118&lt;/a&gt;).&lt;/p&gt;</comment>
                            <comment id="2993948" author="milkie" created="Tue, 24 Mar 2020 12:52:24 +0000"  >&lt;p&gt;Just for clarification, the readOnce flag is not something that will avoid in-memory cache for the pages read.  Instead, it adjusts the score on such pages so that they are evicted from cache sooner than the typical LRU algorithm would.  Because $sample and random cursors potentially read multiple pages per cursor advance, it&apos;s not clear the benefits from the hastening of eviction of such pages would be a good enough tradeoff for the performance hit such cursor reads would experience.&lt;/p&gt;</comment>
                            <comment id="2993944" author="milkie" created="Tue, 24 Mar 2020 12:49:35 +0000"  >&lt;p&gt;Unfortunately, after some discussion with Execution and Storage Engines team members, I believe we&apos;ll have to abandon this change to use readOnce.  The feature is too unstable for us to be comfortable that it will improve performance, and in some cases we have seen that it greatly hurts performance for the scanning cursor itself with no corresponding performance increase for other readers and writers.  The WiredTiger storage engine cache management system is very complex, so it is hard to anticipate what the repercussions of using readOnce cursors will be for all the workloads we are interested in.&lt;/p&gt;</comment>
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            <issuekey id="1290761">SERVER-47118</issuekey>
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