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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-11840] groupcommitwithlimitedlocks doesn&apos;t work</title>
                <link>https://jira.mongodb.org/browse/SERVER-11840</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;groupCommitWithLimitedLocks() isn&apos;t really letting other stuff run in parallel, it appears, as there will be contention on groupCommitMutex even though it&apos;s not locking the db&apos;s themselves.  I will look into it.&lt;/p&gt;</description>
                <environment></environment>
        <key id="99730">SERVER-11840</key>
            <summary>groupcommitwithlimitedlocks doesn&apos;t work</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="kaloian.manassiev@mongodb.com">Kaloian Manassiev</assignee>
                                    <reporter username="dwight@mongodb.com">Dwight Merriman</reporter>
                        <labels>
                    </labels>
                <created>Sat, 23 Nov 2013 16:26:15 +0000</created>
                <updated>Tue, 3 Mar 2015 18:32:57 +0000</updated>
                            <resolved>Thu, 26 Feb 2015 19:13:24 +0000</resolved>
                                    <version>2.5.4</version>
                                    <fixVersion>3.0.0-rc7</fixVersion>
                                    <component>Concurrency</component>
                                        <votes>1</votes>
                                    <watches>7</watches>
                                                                                                                <comments>
                            <comment id="838266" author="kaloian.manassiev" created="Thu, 26 Feb 2015 19:13:24 +0000"  >&lt;p&gt;Fixed in f346c4ad0c2d892014c79d7adbbca61031f50194.&lt;/p&gt;</comment>
                            <comment id="837994" author="fdoray" created="Thu, 26 Feb 2015 15:27:44 +0000"  >&lt;p&gt;Pull request submitted here &lt;a href=&quot;https://github.com/mongodb/mongo/pull/929&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/pull/929&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="837781" author="fdoray" created="Thu, 26 Feb 2015 06:22:33 +0000"  >&lt;p&gt;I executed many consecutive writes from a simple C++ application (&lt;a href=&quot;https://github.com/fdoray/trace-kit/blob/master/src/apps/mongowrite.cpp&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/fdoray/trace-kit/blob/master/src/apps/mongowrite.cpp&lt;/a&gt;) and measured their duration. The results are here: &lt;a href=&quot;https://github.com/fdoray/trace-kit/tree/master/results/mongowrite&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/fdoray/trace-kit/tree/master/results/mongowrite&lt;/a&gt; (withoutfix.csv and withoutfix.svg). We can see that at regular intervals, the write operations take more time.&lt;/p&gt;

&lt;p&gt;Using a critical path analysis(using LTTng), I was able to determine that the slow writes were caused by the fact that groupCommitMutex was held during the call to WRITETOJOURNAL (&lt;a href=&quot;http://fdoray.github.io/tracecompare/?data=mongowrite&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;http://fdoray.github.io/tracecompare/?data=mongowrite&lt;/a&gt;).&lt;/p&gt;

&lt;p&gt;I prepared a fix here: &lt;a href=&quot;https://github.com/fdoray/mongo/commit/4e5711e571b67a6774ac36abd6bfdb59ee1c913d&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/fdoray/mongo/commit/4e5711e571b67a6774ac36abd6bfdb59ee1c913d&lt;/a&gt; . The long writes that we had before are no longer there (withfix.csv and withfix.svg). (Note that there is still a few long writes, but recording a trace with LTTng shows that they are caused by dirty pages being written to disk and not by lock contention).&lt;/p&gt;

&lt;p&gt;I will check that my assumptions about how locking work within MongoDB are correct and send a pull request as soon as possible.&lt;/p&gt;</comment>
                            <comment id="461987" author="dwight_10gen" created="Tue, 26 Nov 2013 14:55:11 +0000"  >&lt;p&gt;more detail: if a writer tries to get groupCommitMutex in the old code (which it will), it will be blocked until the commit finishes completely.  that will then block even readers (although greedy behavior may be problematic thereof regardless).&lt;/p&gt;

&lt;p&gt;if fixed, you can then also write during the WRITETOJOURNAL and WRITETODATAFILES phases of a LimitedLocks commit.&lt;/p&gt;</comment>
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