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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-78528] Background threads in mongos cause mongos_large_catalog_workloads test case to measure performance incorrectly</title>
                <link>https://jira.mongodb.org/browse/SERVER-78528</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;It was observed from analysis of &lt;a href=&quot;https://jira.mongodb.org/browse/BF-28472&quot; class=&quot;external-link&quot; rel=&quot;nofollow&quot;&gt;BF-28472&lt;/a&gt; that the introduction of using transactions in &lt;tt&gt;ClusterServerParameterRefresher&lt;/tt&gt; thread and the &lt;tt&gt;AuditSyncJob&lt;/tt&gt; thread as part of &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-74107&quot; class=&quot;external-link&quot; rel=&quot;nofollow&quot;&gt;SERVER-74107&lt;/a&gt; cause timing issues when measuring performance in &lt;a href=&quot;https://github.com/10gen/workloads/blob/77693ff9d646ba04f8ea13c7f7c967ba45da4f42/workloads/mongos_large_catalog.js#L294&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;mongos_large_catalog_workloads&lt;/a&gt;, specifically for &lt;tt&gt;1000_colls_fresh_mongos&lt;/tt&gt; test case. It appears that the scheduling of these threads affects latency of some operations enough to cause a regression. A couple patches(&lt;a href=&quot;https://spruce.mongodb.com/version/649b884961837d957c4e21a8/tasks&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;patch1&lt;/a&gt;, &lt;a href=&quot;https://spruce.mongodb.com/version/649b884bd6d80af442078776/tasks&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;patch2&lt;/a&gt;) were run for to prove this with results in screenshot attached. &lt;span class=&quot;image-wrap&quot; style=&quot;&quot;&gt;&lt;a id=&quot;459166_thumb&quot; href=&quot;https://jira.mongodb.org/secure/attachment/459166/459166_Screenshot+2023-06-28+at+1.47.20+PM.png&quot; title=&quot;Screenshot 2023-06-28 at 1.47.20 PM.png&quot; file-preview-type=&quot;image&quot; file-preview-id=&quot;459166&quot; file-preview-title=&quot;Screenshot 2023-06-28 at 1.47.20 PM.png&quot;&gt;&lt;img src=&quot;https://jira.mongodb.org/secure/thumbnail/459166/_thumb_459166.png&quot; style=&quot;border: 0px solid black&quot; role=&quot;presentation&quot;/&gt;&lt;/a&gt;&lt;/span&gt; &lt;/p&gt;

&lt;p&gt;One possible fix is to disable these threads from running when measuring performance. &lt;/p&gt;</description>
                <environment></environment>
        <key id="2379841">SERVER-78528</key>
            <summary>Background threads in mongos cause mongos_large_catalog_workloads test case to measure performance incorrectly</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="3">Duplicate</resolution>
                                        <assignee username="backlog-server-security">Backlog - Security Team</assignee>
                                    <reporter username="abdul.qadeer@mongodb.com">Abdul Qadeer</reporter>
                        <labels>
                    </labels>
                <created>Wed, 28 Jun 2023 20:52:06 +0000</created>
                <updated>Mon, 10 Jul 2023 20:25:19 +0000</updated>
                            <resolved>Mon, 10 Jul 2023 20:25:19 +0000</resolved>
                                                                                        <votes>0</votes>
                                    <watches>2</watches>
                                                                                                                <comments>
                            <comment id="5555753" author="JIRAUSER1253409" created="Mon, 10 Jul 2023 20:25:19 +0000"  >&lt;p&gt;Closing this as a duplicate because these two tickets are pointing at the same issue.&lt;/p&gt;</comment>
                            <comment id="5549804" author="JIRAUSER1265450" created="Fri, 7 Jul 2023 09:54:13 +0000"  >&lt;p&gt;The proper way to fix this issue would be to disable the clusterParameterRefresh. However we only have one failpoint to do so and it is only implemented in master. The workload project it&apos;s independent on the version of mongod running, meaning triggering the failpoint would fail for versions&#160; &amp;lt;=7.0.&#160;&lt;/p&gt;

&lt;p&gt;I am thinking the only way to properly fix this would be to implement an outlier detector. The 60% regression detected in&#160;BF-28894&#160;is on the average of 5 runs. By looking at the logs, the actual issue is only on 1 run (the first) with a 400% regression followed by 4 runs around the same speed and in line with pre-regression runs. This is probably due to the overlap with&#160; the clusterParameterRefresh that before the revert used to serialise with the refresh for the fresh mongos.&#160; However, excluding outliers might in future prevent us from spotting real issues, which should never happen. I would be curious to know what performance think about that.&lt;/p&gt;

&lt;p&gt;Another option it&apos;s to simply accept the result. It only depends what the result means and what we want to measure. The background process in a real scenario will be enabled and might overlap with the refresh at any time. So the average it&apos;s actually correct.&apos;&lt;/p&gt;

&lt;p&gt;In general, we are talking about 2ms slower refresh over 100k collections, which is still a negligible drop in performance. The refresh stays still very fast. From sharding I would personally close the ticket as &quot;won&apos;t fix&quot;, however I will assign it back to Security so they can evaluate whether it&apos;s worth reintroduce the failpoint for 7.0&#160;&lt;/p&gt;</comment>
                    </comments>
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                            <attachment id="459166" name="Screenshot 2023-06-28 at 1.47.20 PM.png" size="284935" author="abdul.qadeer@mongodb.com" created="Wed, 28 Jun 2023 20:47:32 +0000"/>
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