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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-10362] yielding during read queries waiting too long for fair locking</title>
                <link>https://jira.mongodb.org/browse/SERVER-10362</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;div class=&quot;panel&quot; style=&quot;background-color: #EEEEEE;border-color: #ccc;border-width: 1px;&quot;&gt;&lt;div class=&quot;panelHeader&quot; style=&quot;border-bottom-width: 1px;border-bottom-color: #ccc;background-color: #6CB33F;&quot;&gt;&lt;b&gt;MongoDB Status as of January 3rd, 2014&lt;/b&gt;&lt;/div&gt;&lt;div class=&quot;panelContent&quot; style=&quot;background-color: #EEEEEE;&quot;&gt;
&lt;p&gt;&lt;b&gt;ISSUE SUMMARY&lt;/b&gt;&lt;/p&gt;

&lt;p&gt;Starting in version 2.4, a call to sleepmicros(1) was added to the yielding logic which is called after scanning each 256 documents when finding the start position for the tailable cursor used in replication.  On some hypervisors, the sleepmicros() call actually sleeps for much more than a microsecond (its guaranteed to &quot;sleep at least 1 microsecond&quot;).  It can sleep as long as one millisecond or longer, at times.&lt;/p&gt;

&lt;p&gt;&lt;b&gt;USER IMPACT&lt;/b&gt;&lt;br/&gt;
The change to the yielding logic on certain hypervisors can affect the method &quot;FindingStartCursor&quot;, which is the method that replica sets use to find their position in their sync source&apos;s oplog.  On a busy system this query could take a lot longer than it should have, sometimes resulting in timeouts.&lt;/p&gt;

&lt;p&gt;&lt;b&gt;SOLUTION&lt;/b&gt;&lt;br/&gt;
Replace the call to sleepmicros(1) with a call to pthread_yield() on linux.&lt;/p&gt;

&lt;p&gt;&lt;b&gt;WORKAROUNDS&lt;/b&gt;&lt;br/&gt;
There is no workaround.&lt;/p&gt;

&lt;p&gt;&lt;b&gt;PATCHES&lt;/b&gt;&lt;br/&gt;
Production release v2.4.6 contains the fix for this issue, and production release v2.6.0 will contain the fix as well.&lt;/p&gt;
&lt;/div&gt;&lt;/div&gt;

&lt;h4&gt;&lt;a name=&quot;OriginalDescription&quot;&gt;&lt;/a&gt;Original Description&lt;/h4&gt;

&lt;p&gt;In 2.4, sleepmicros(1) was added to the yield code which is called after scanning 256 documents when finding the start position for the tailable cursor used in replication.&lt;br/&gt;
On some platforms (in particular, virtualized ones), the sleepmicros() call actually sleeps for much more than a microsecond (its guaranteed to &quot;sleep at least 1 microsecond&quot;)&lt;br/&gt;
It can sleep as long as one millisecond or longer, at times.&lt;/p&gt;

&lt;p&gt;This affected FindingStartCursor particularly hard, which is the method that replica sets use to find their position in their sync source&apos;s oplog.  Thus on a busy system this query was taking a lot longer than it should have, sometimes resulting in timeouts.&lt;/p&gt;</description>
                <environment></environment>
        <key id="83711">SERVER-10362</key>
            <summary>yielding during read queries waiting too long for fair locking</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="eliot">Eliot Horowitz</assignee>
                                    <reporter username="dan@mongodb.com">Daniel Pasette</reporter>
                        <labels>
                    </labels>
                <created>Sat, 27 Jul 2013 18:07:03 +0000</created>
                <updated>Mon, 11 Jul 2016 17:37:35 +0000</updated>
                            <resolved>Tue, 30 Jul 2013 13:51:44 +0000</resolved>
                                    <version>2.4.0</version>
                                    <fixVersion>2.4.6</fixVersion>
                    <fixVersion>2.5.2</fixVersion>
                                    <component>Concurrency</component>
                    <component>Replication</component>
                                        <votes>0</votes>
                                    <watches>8</watches>
                                                                                                                <comments>
                            <comment id="415121" author="auto" created="Fri, 30 Aug 2013 19:07:40 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{u&apos;username&apos;: u&apos;milkie&apos;, u&apos;name&apos;: u&apos;Eric Milkie&apos;, u&apos;email&apos;: u&apos;milkie@10gen.com&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-10362&quot; title=&quot;yielding during read queries waiting too long for fair locking&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-10362&quot;&gt;&lt;del&gt;SERVER-10362&lt;/del&gt;&lt;/a&gt; actually sleep when writers yield&lt;br/&gt;
Branch: master&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/commit/c0d3a2fc9969c36045c5b753a3a7a7a26bb990be&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/c0d3a2fc9969c36045c5b753a3a7a7a26bb990be&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="394106" author="auto" created="Fri, 2 Aug 2013 01:03:29 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{u&apos;username&apos;: u&apos;monkey101&apos;, u&apos;name&apos;: u&apos;Dan Pasette&apos;, u&apos;email&apos;: u&apos;dan@10gen.com&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-10362&quot; title=&quot;yielding during read queries waiting too long for fair locking&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-10362&quot;&gt;&lt;del&gt;SERVER-10362&lt;/del&gt;&lt;/a&gt;: call ClientCursor::staticYield to do a pthread_yield rather than sleepmicros.&lt;br/&gt;
Branch: v2.4&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/commit/d502de0e89f72e6a912d68dfa34b14354d3210d1&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/d502de0e89f72e6a912d68dfa34b14354d3210d1&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="394099" author="auto" created="Fri, 2 Aug 2013 00:48:57 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{u&apos;username&apos;: u&apos;erh&apos;, u&apos;name&apos;: u&apos;Eliot Horowitz&apos;, u&apos;email&apos;: u&apos;eliot@10gen.com&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-10362&quot; title=&quot;yielding during read queries waiting too long for fair locking&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-10362&quot;&gt;&lt;del&gt;SERVER-10362&lt;/del&gt;&lt;/a&gt;: only use pthread_yield on linux, on osx use sleepmicros(1)&lt;br/&gt;
Branch: v2.4&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/commit/12ecb7dbc765df9544d21bb3823a5fbde0efe0e6&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/12ecb7dbc765df9544d21bb3823a5fbde0efe0e6&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="394098" author="auto" created="Fri, 2 Aug 2013 00:48:55 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{u&apos;username&apos;: u&apos;erh&apos;, u&apos;name&apos;: u&apos;Eliot Horowitz&apos;, u&apos;email&apos;: u&apos;eliot@10gen.com&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-10362&quot; title=&quot;yielding during read queries waiting too long for fair locking&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-10362&quot;&gt;&lt;del&gt;SERVER-10362&lt;/del&gt;&lt;/a&gt;: add option to ClientCursor::staticYield to do a pthread_yield instead of sleepmicros&lt;br/&gt;
              this is mostly because of some platforms (like xen) where sleepmicros is too inaccurate&lt;br/&gt;
Branch: v2.4&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/commit/6d738db1f45ec0f7447e2a14e0216af2410074c8&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/6d738db1f45ec0f7447e2a14e0216af2410074c8&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="391236" author="auto" created="Mon, 29 Jul 2013 19:51:12 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{u&apos;username&apos;: u&apos;erh&apos;, u&apos;name&apos;: u&apos;Eliot Horowitz&apos;, u&apos;email&apos;: u&apos;eliot@10gen.com&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-10362&quot; title=&quot;yielding during read queries waiting too long for fair locking&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-10362&quot;&gt;&lt;del&gt;SERVER-10362&lt;/del&gt;&lt;/a&gt;: only use pthread_yield on linux, on osx use sleepmicros(1)&lt;br/&gt;
Branch: master&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/commit/9498d1148cb14c862b615a528e65aa757705aa00&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/9498d1148cb14c862b615a528e65aa757705aa00&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="391166" author="auto" created="Mon, 29 Jul 2013 18:47:21 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{u&apos;username&apos;: u&apos;erh&apos;, u&apos;name&apos;: u&apos;Eliot Horowitz&apos;, u&apos;email&apos;: u&apos;eliot@10gen.com&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-10362&quot; title=&quot;yielding during read queries waiting too long for fair locking&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-10362&quot;&gt;&lt;del&gt;SERVER-10362&lt;/del&gt;&lt;/a&gt;: tell ClientCursor::staticYield to do a pthread_yield rather than sleepmicros&lt;br/&gt;
              this ensures this call doesn&apos;t take too long on some platforms (xen)&lt;br/&gt;
Branch: master&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/commit/11f47af7789e8825d92bbcf76b6930d2962fbb7e&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/11f47af7789e8825d92bbcf76b6930d2962fbb7e&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="391165" author="auto" created="Mon, 29 Jul 2013 18:47:19 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{u&apos;username&apos;: u&apos;erh&apos;, u&apos;name&apos;: u&apos;Eliot Horowitz&apos;, u&apos;email&apos;: u&apos;eliot@10gen.com&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-10362&quot; title=&quot;yielding during read queries waiting too long for fair locking&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-10362&quot;&gt;&lt;del&gt;SERVER-10362&lt;/del&gt;&lt;/a&gt;: add option to ClientCursor::staticYield to do a pthread_yield instead of sleepmicros&lt;br/&gt;
              this is mostly because of some platforms (like xen) where sleepmicros is too inaccurate&lt;br/&gt;
Branch: master&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/commit/d2ce20775779a3116f693b9e60744234bdbc5f50&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/d2ce20775779a3116f693b9e60744234bdbc5f50&lt;/a&gt;&lt;/p&gt;</comment>
                    </comments>
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            <issuekey id="68048">SERVER-8939</issuekey>
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            <issuekey id="75534">SERVER-9707</issuekey>
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