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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>[DOCS-11746] Wrong swappiness configuration recommendation</title>
                <link>https://jira.mongodb.org/browse/DOCS-11746</link>
                <project id="10380" key="DOCS">Documentation</project>
                    <description>&lt;p&gt;I have found today that the official recommendation is to set &lt;a href=&quot;https://docs.mongodb.com/manual/administration/production-notes/#set-vm-swappiness-to-1&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;vm.swappiness to 1&lt;/a&gt;.&lt;/p&gt;

&lt;p&gt;I do believe this is a serious misunderstanding of the purpose of the swap space. Apparently, the documentation was worse (set to 0) as I can see on&#160;&lt;a href=&quot;https://jira.mongodb.org/browse/DOCS-10405&quot; title=&quot;Add swappiness&amp;gt;0 recommendation to production notes&quot; class=&quot;issue-link&quot; data-issue-key=&quot;DOCS-10405&quot;&gt;&lt;del&gt;DOCS-10405&lt;/del&gt;&lt;/a&gt;.&lt;/p&gt;

&lt;p&gt;From the official &lt;a href=&quot;https://www.kernel.org/doc/gorman/html/understand/understand014.html#note15&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;Linux documentation&lt;/a&gt; you can read the following:&lt;/p&gt;
&lt;blockquote&gt;&lt;p&gt;The casual reader^&lt;a href=&quot;https://www.kernel.org/doc/gorman/html/understand/understand014.html#note15&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;1&lt;/a&gt;^&#160;may think that with a sufficient amount of memory, swap is unnecessary but this brings us to the second reason. A significant number of the pages referenced by a process early in its life may only be used for initialisation and then never used again. It is better to swap out those pages and create more disk buffers than leave them resident and unused.&lt;/p&gt;&lt;/blockquote&gt;
&lt;p&gt;The swap usage is not a problem by itself. The only problem which may affect performance is if the operating system is actively paging&#160;in and out which can be tracked using &lt;b&gt;vmstat&lt;/b&gt;.&lt;/p&gt;

&lt;p&gt;You can find many articles in relation to this subject:&lt;/p&gt;

&lt;p&gt;&lt;a href=&quot;https://chrisdown.name/2018/01/02/in-defence-of-swap.html&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://chrisdown.name/2018/01/02/in-defence-of-swap.html&lt;/a&gt;&lt;/p&gt;

&lt;p&gt;In the previous documentation ticket, the value of 10 was suggested which is better than 1. However, I would not recommend changing anything unless you really know what you are doing and you have the tools to observe the behavior and the impact.&lt;/p&gt;

&lt;p&gt;&#160;&lt;/p&gt;
&lt;h2&gt;&lt;a name=&quot;Scope&quot;&gt;&lt;/a&gt;Scope&lt;/h2&gt;

&lt;p&gt;Based on internal discussions, we&apos;re going to remove this recommendation until performance testing completes and we have more data to back any recommendations in one direction or another. We will backport this removal to 3.6.&lt;/p&gt;</description>
                <environment></environment>
        <key id="550599">DOCS-11746</key>
            <summary>Wrong swappiness configuration recommendation</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="12300">Won&apos;t Do</resolution>
                                        <assignee username="ravind.kumar">Ravind Kumar</assignee>
                                    <reporter username="ricardo.lorenzo">Ricardo Lorenzo</reporter>
                        <labels>
                    </labels>
                <created>Tue, 29 May 2018 10:56:57 +0000</created>
                <updated>Mon, 30 Oct 2023 20:58:28 +0000</updated>
                                                            <fixVersion>Server_Docs_20231030</fixVersion>
                                    <component>manual</component>
                    <component>Server</component>
                        <due></due>
                            <votes>1</votes>
                                    <watches>17</watches>
                                                                                                                <comments>
                            <comment id="4940585" author="edu.bot" created="Mon, 31 Oct 2022 16:07:49 +0000"  >&lt;p&gt;Hello! This ticket has been closed due to inactivity. If you believe this ticket is still important, please reopen it and leave a comment to explain why. Thank you!&lt;/p&gt;</comment>
                            <comment id="2125438" author="henrik.ingo@10gen.com" created="Thu, 24 Jan 2019 05:39:25 +0000"  >&lt;p&gt;I didn&apos;t want to suggest more than one alternative, but for the record, I am not against removing this recommendation entirely.&lt;/p&gt;</comment>
                            <comment id="2111064" author="bruce.lucas@10gen.com" created="Thu, 10 Jan 2019 15:03:18 +0000"  >&lt;blockquote&gt;
&lt;p&gt;if some other thing suddenly uses up more memory than you would expect, mongod can end up getting killed by the OOM thread&lt;/p&gt;&lt;/blockquote&gt;
&lt;p&gt;Not to mention if mongod itself uses much more memory than expected, and there are a number of known reasons this can happen.&lt;/p&gt;

&lt;p&gt;However it seems debatable to me whether it&apos;s better to start paging and run really slowly for a long time, or fall over and restart quickly in this scenario.&lt;/p&gt;</comment>
                            <comment id="2109795" author="henrik.ingo@10gen.com" created="Wed, 9 Jan 2019 14:14:20 +0000"  >&lt;p&gt;I believe Atlas tested Ricardo&apos;s recommendation on the smaller instances, but &quot;felt&quot; that if anything, it made things worse.&lt;/p&gt;

&lt;p&gt;Cailin said this in an email once. I&apos;m intentionally not pinging her, as she probably has more important things to focus on today. But someone could ask Cloud team about this at a later time.&lt;/p&gt;</comment>
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