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<channel>
    <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>
    </build-info>


<item>
            <title>[SERVER-24631] TTL Monitor performance degradation on MongoDB 3.0</title>
                <link>https://jira.mongodb.org/browse/SERVER-24631</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;Hi,&lt;/p&gt;

&lt;p&gt;I&#8217;ve noticed a severe spike in resource utilization leading to degradation in performance on AWS since upgrading to 3.0 as a result of the way the &lt;tt&gt;TTLMonitor&lt;/tt&gt; queries for indexes when using &lt;tt&gt;MMAPv1&lt;/tt&gt;.&lt;/p&gt;

&lt;p&gt;In 2.6, TTL indexes were collected by querying the &lt;tt&gt;system.indexes&lt;/tt&gt; collection like so:&lt;/p&gt;

&lt;p/&gt;
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					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;  margin-top: 10px;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;                auto_ptr&amp;lt;DBClientCursor&amp;gt; cursor =&lt;/span&gt;&lt;/pre&gt;
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					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;                                db.query( dbName + &lt;/span&gt;&lt;span style=&quot;color: blue; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;&quot;.system.indexes&quot;&lt;/span&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt; ,&lt;/span&gt;&lt;/pre&gt;
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					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;                                          BSON( secondsExpireField &amp;lt;&amp;lt; BSON( &lt;/span&gt;&lt;span style=&quot;color: blue; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;&quot;$exists&quot;&lt;/span&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt; &amp;lt;&amp;lt; &lt;/span&gt;&lt;span style=&quot;color: #006699; font-weight: bold; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;true&lt;/span&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt; ) ) ,&lt;/span&gt;&lt;/pre&gt;
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					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   margin-bottom: 10px;  width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;                                          QueryOption_SlaveOk ); &lt;/span&gt;&lt;span style=&quot;color: #008200; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;/* perform on secondaries too */&lt;/span&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;&lt;/span&gt;&lt;/pre&gt;
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&lt;p/&gt;

&lt;p&gt;In 3.0 &lt;tt&gt;system.indexes&lt;/tt&gt; was deprecated and a new abstraction layer between the database and the storage engine was introduced. As a result, namespace file operations are now much less efficient. The issue I am seeing appears to be the result of the following bit of code:&lt;/p&gt;

&lt;p/&gt;
&lt;div id=&quot;syntaxplugin&quot; class=&quot;syntaxplugin&quot; style=&quot;border: 1px dashed #bbb; border-radius: 5px !important; overflow: auto; max-height: 30em;&quot;&gt;
&lt;table cellspacing=&quot;0&quot; cellpadding=&quot;0&quot; border=&quot;0&quot; width=&quot;100%&quot; style=&quot;font-size: 1em; line-height: 1.4em !important; font-weight: normal; font-style: normal; color: black;&quot;&gt;
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					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;  margin-top: 10px;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;    &lt;/span&gt;&lt;span style=&quot;color: #006699; font-weight: bold; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;void&lt;/span&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt; iterAll(IteratorCallback callback) {&lt;/span&gt;&lt;/pre&gt;
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					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;        &lt;/span&gt;&lt;span style=&quot;color: #006699; font-weight: bold; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;for&lt;/span&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt; (&lt;/span&gt;&lt;span style=&quot;color: #006699; font-weight: bold; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;int&lt;/span&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt; i = 0; i &amp;lt; n; i++) {&lt;/span&gt;&lt;/pre&gt;
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					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;            &lt;/span&gt;&lt;span style=&quot;color: #006699; font-weight: bold; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;if&lt;/span&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt; (_nodes(i).inUse()) {&lt;/span&gt;&lt;/pre&gt;
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&lt;p/&gt;

&lt;p&gt;which gets executed by the &lt;tt&gt;TTLMonitor&lt;/tt&gt; via this code path:&lt;/p&gt;

&lt;p/&gt;
&lt;div id=&quot;syntaxplugin&quot; class=&quot;syntaxplugin&quot; style=&quot;border: 1px dashed #bbb; border-radius: 5px !important; overflow: auto; max-height: 30em;&quot;&gt;
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					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;  margin-top: 10px;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;mongo/db/storage/mmap_v1/catalog/hashtab.h - NamespaceHashTable::iterAll&lt;/span&gt;&lt;/pre&gt;
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					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;mongo/db/storage/mmap_v1/catalog/namespace_index.cpp - NamespaceIndex::getCollectionNamespaces&lt;/span&gt;&lt;/pre&gt;
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&lt;p/&gt;

&lt;p&gt;As a result, the entire namespace file for every database gets pulled into memory &lt;b&gt;every&lt;/b&gt; time the &lt;tt&gt;TTLMonitor&lt;/tt&gt; executes (every 60 seconds by default). Of course, the default namespace file size is only 16MBs, so this really isn&#8217;t an issue in the most common case. However, if you want to set up a development environment for a number of users on a single host, you will find yourself scratching your head as to why performance is so bad. It should be noted that performance will be bad regardless of user activity, database size, or the presence of TTL indexes (all of which would only serve to exacerbate the situation).&lt;/p&gt;

&lt;p&gt;In addition to the development use case, it is possible to run into similar issues with a single database that has many collections/indexes and requires a namespace file larger than the default (up to 2048 MBs). In this case, both the &lt;tt&gt;TTLMonito&lt;/tt&gt; and any command that involves a namespace file scan (e.g., &lt;tt&gt;listCollections&lt;/tt&gt;) will cause issues.&lt;/p&gt;

&lt;p&gt;To reproduce this behavior, you can spin up a &lt;tt&gt;t2.small&lt;/tt&gt; instance on AWS, attach a 33 GB &lt;tt&gt;gp2&lt;/tt&gt; volume, deploy MongoDB 3.0.12, ensure that dbpath points to a a directory that lives on the attached volume, and, finally, restore the attached dump. You should see read behavior on that volume that resembles the attached graphs and &lt;tt&gt;vmstat&lt;/tt&gt; dump with &lt;b&gt;zero&lt;/b&gt; user activity.&lt;/p&gt;

&lt;p&gt;At the very least, I think this should be explicitly documented in order to save time/confusion on the part of developers/operations and to help with capacity planning and architecture decisions going forward.&lt;/p&gt;

&lt;p&gt;Greg&lt;/p&gt;</description>
                <environment>AWS t2.small&lt;br/&gt;
$&amp;gt; uname -a&lt;br/&gt;
Linux ip-10-0-0-121 3.13.0-74-generic #118-Ubuntu SMP Thu Dec 17 22:52:10 UTC 2015 x86_64 x86_64 x86_64 GNU/Linux&lt;br/&gt;
$&amp;gt; cat /etc/lsb-release&lt;br/&gt;
DISTRIB_ID=Ubuntu&lt;br/&gt;
DISTRIB_RELEASE=14.04&lt;br/&gt;
DISTRIB_CODENAME=trusty&lt;br/&gt;
DISTRIB_DESCRIPTION=&amp;quot;Ubuntu 14.04.4 LTS&amp;quot;</environment>
        <key id="294621">SERVER-24631</key>
            <summary>TTL Monitor performance degradation on MongoDB 3.0</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="9">Done</resolution>
                                        <assignee username="kevin.albertson@mongodb.com">Kevin Albertson</assignee>
                                    <reporter username="gregbanks">Gregory Banks</reporter>
                        <labels>
                    </labels>
                <created>Fri, 17 Jun 2016 02:11:36 +0000</created>
                <updated>Mon, 30 Jan 2017 15:04:17 +0000</updated>
                            <resolved>Tue, 19 Jul 2016 18:01:21 +0000</resolved>
                                    <version>3.0.12</version>
                                    <fixVersion>3.3.11</fixVersion>
                                    <component>MMAPv1</component>
                                        <votes>0</votes>
                                    <watches>12</watches>
                                                                                                                <comments>
                            <comment id="1328685" author="xgen-internal-githook" created="Tue, 19 Jul 2016 17:59:15 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{u&apos;username&apos;: u&apos;kevinAlbs&apos;, u&apos;name&apos;: u&apos;Kevin Albertson&apos;, u&apos;email&apos;: u&apos;kevin.albertson@10gen.com&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-24631&quot; title=&quot;TTL Monitor performance degradation on MongoDB 3.0&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-24631&quot;&gt;&lt;del&gt;SERVER-24631&lt;/del&gt;&lt;/a&gt;: Add TTL collection namespace cache&lt;br/&gt;
Branch: master&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/commit/d059552b998bd9f3ff0275016dff2df89a137b02&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/d059552b998bd9f3ff0275016dff2df89a137b02&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="1304360" author="gregbanks" created="Thu, 23 Jun 2016 16:39:51 +0000"  >&lt;p&gt;Hey Thomas,&lt;/p&gt;

&lt;p&gt;Awesome and no problem &lt;img class=&quot;emoticon&quot; src=&quot;https://jira.mongodb.org/images/icons/emoticons/smile.png&quot; height=&quot;16&quot; width=&quot;16&quot; align=&quot;absmiddle&quot; alt=&quot;&quot; border=&quot;0&quot;/&gt; Thanks!&lt;/p&gt;

&lt;p&gt;Cheers,&lt;br/&gt;
Greg&lt;/p&gt;</comment>
                            <comment id="1301808" author="thomas.schubert" created="Tue, 21 Jun 2016 20:04:35 +0000"  >&lt;p&gt;Hi &lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=gregbanks&quot; class=&quot;user-hover&quot; rel=&quot;gregbanks&quot;&gt;gregbanks&lt;/a&gt;,&lt;/p&gt;

&lt;p&gt;I&apos;ve confirmed that this issue affects MongoDB 3.2.7 as well, and I&apos;m marking this ticket to be scheduled. Please continue to watch for updates.&lt;/p&gt;

&lt;p&gt;Thank you again for the detailed steps to reproduce!&lt;br/&gt;
Thomas&lt;/p&gt;</comment>
                            <comment id="1298203" author="gregbanks" created="Fri, 17 Jun 2016 18:05:53 +0000"  >&lt;p&gt;No problem. Please let me know if you need anything else!&lt;/p&gt;</comment>
                            <comment id="1297765" author="ramon.fernandez" created="Fri, 17 Jun 2016 13:36:52 +0000"  >&lt;p&gt;Thanks for the detailed bug report &lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=gregbanks&quot; class=&quot;user-hover&quot; rel=&quot;gregbanks&quot;&gt;gregbanks&lt;/a&gt;, we&apos;re investigating.&lt;/p&gt;</comment>
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