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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-51041] Throttle starting transactions for secondary reads</title>
                <link>https://jira.mongodb.org/browse/SERVER-51041</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;&lt;b&gt;This performance regression affects readConcern &quot;local&quot; and &quot;available&quot; reads on secondary nodes.&lt;/b&gt;&lt;/p&gt;

&lt;p&gt;&lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-46721&quot; title=&quot;Step up may cause reads at PIT with holes after yielding&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-46721&quot;&gt;&lt;del&gt;SERVER-46721&lt;/del&gt;&lt;/a&gt;&#160;removed a mutex around a critical section that effectively synchronized every external secondary reader that reads at lastApplied. I deemed &lt;a href=&quot;https://github.com/mongodb/mongo/commit/caef6619556ddeb20adf8e2bad651b96d2b3ea70#diff-4627e7f75a6bd0db7f8f46d9e4f7dca3L102-L106&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;this mutex&lt;/a&gt;&#160;unnecessary, but removing it pushed a synchronization problem down to a lower level.&lt;/p&gt;

&lt;p&gt;For high volumes of short-lived secondary reads, it appears as though the WT reader-writer lock for the global read timestamp queue does not handle excessive contention as well as the mutex before it.&lt;/p&gt;

&lt;p&gt;The problem I see is that the WT read timestamp queue leaves around old entries from inactive transactions. New readers (holding write locks on the read timestamp queue) are responsible for &lt;a href=&quot;https://github.com/mongodb/mongo/blob/06402114114ffc5146fd4b55402c96f1dc9ec4b5/src/third_party/wiredtiger/src/txn/txn_timestamp.c#L1171-L1182&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;cleaning up old entries&lt;/a&gt; even if the queue has hundreds of thousands of inactive entries. This then blocks out other readers, which spin wait for a moment, then start context switching wildly. Once the queue shrinks down, thousands of new read requests come in, but the problem just repeats itself. This leads to very unpredicatable latencies and poor CPU utilization.&lt;/p&gt;

&lt;p&gt;I was able to fix the performance problem by re-introducing a mutex around the area where we start transactions for secondary reads (at lastApplied):&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;diff --git a/src/mongo/db/storage/wiredtiger/wiredtiger_recovery_unit.cpp b/src/mongo/db/storage/wiredtiger/wiredtiger_recovery_unit.cpp&lt;/span&gt;&lt;/pre&gt;
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						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&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;index 3f07c244c5..bcdef7f70c 100644&lt;/span&gt;&lt;/pre&gt;
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				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: #2B91AF; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;--- a/src/mongo/db/storage/wiredtiger/wiredtiger_recovery_unit.cpp&lt;/span&gt;&lt;/pre&gt;
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				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: #2B91AF; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;+++ b/src/mongo/db/storage/wiredtiger/wiredtiger_recovery_unit.cpp&lt;/span&gt;&lt;/pre&gt;
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						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&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;@@ -590,6 +590,7 @@ Timestamp WiredTigerRecoveryUnit::_beginTransactionAtAllDurableTimestamp(WT_SESS&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
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						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: gray; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;     return readTimestamp;&lt;/span&gt;&lt;/pre&gt;
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						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: gray; 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;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: gray; 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;
			&lt;/td&gt;
		&lt;/tr&gt;
				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   width: auto; padding: 0;&quot;&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;+Mutex _lastAppliedTxnMutex = MONGO_MAKE_LATCH(&quot;_lastAppliedTxnMutex&quot;);&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: gray; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt; Timestamp WiredTigerRecoveryUnit::_beginTransactionAtLastAppliedTimestamp(WT_SESSION* session) {&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: gray; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;     auto lastApplied = _sessionCache-&amp;gt;snapshotManager().getLastApplied();&lt;/span&gt;&lt;/pre&gt;
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		&lt;/tr&gt;
				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: gray; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;     if (!lastApplied) {&lt;/span&gt;&lt;/pre&gt;
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		&lt;/tr&gt;
				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&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;@@ -609,6 +610,8 @@ Timestamp WiredTigerRecoveryUnit::_beginTransactionAtLastAppliedTimestamp(WT_SES&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
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						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: gray; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;         return Timestamp();&lt;/span&gt;&lt;/pre&gt;
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						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: gray; 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;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: gray; 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;
			&lt;/td&gt;
		&lt;/tr&gt;
				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   width: auto; padding: 0;&quot;&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;+&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   width: auto; padding: 0;&quot;&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;+    stdx::lock_guard&amp;lt;Latch&amp;gt; lock(_lastAppliedTxnMutex);&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: gray; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;     WiredTigerBeginTxnBlock txnOpen(session,&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: gray; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;                                     _prepareConflictBehavior,&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   margin-bottom: 10px;  width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: gray; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;                                     _roundUpPreparedTimestamps,&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;
			&lt;/td&gt;
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			&lt;/tbody&gt;
&lt;/table&gt;
&lt;/div&gt;
&lt;p/&gt;</description>
                <environment></environment>
        <key id="1480060">SERVER-51041</key>
            <summary>Throttle starting transactions for secondary reads</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="13201">Fixed</resolution>
                                        <assignee username="louis.williams@mongodb.com">Louis Williams</assignee>
                                    <reporter username="louis.williams@mongodb.com">Louis Williams</reporter>
                        <labels>
                            <label>KP44</label>
                    </labels>
                <created>Fri, 18 Sep 2020 14:09:30 +0000</created>
                <updated>Wed, 10 Jan 2024 22:53:09 +0000</updated>
                            <resolved>Mon, 21 Sep 2020 14:01:14 +0000</resolved>
                                    <version>4.2.9</version>
                    <version>4.4.1</version>
                    <version>4.7.0</version>
                                    <fixVersion>4.8.0</fixVersion>
                    <fixVersion>4.2.10</fixVersion>
                    <fixVersion>4.4.2</fixVersion>
                                                        <votes>0</votes>
                                    <watches>19</watches>
                                                                                                                <comments>
                            <comment id="3403803" author="xgen-internal-githook" created="Mon, 21 Sep 2020 18:46:10 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{&apos;name&apos;: &apos;Louis Williams&apos;, &apos;email&apos;: &apos;louis.williams@mongodb.com&apos;, &apos;username&apos;: &apos;louiswilliams&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-51041&quot; title=&quot;Throttle starting transactions for secondary reads&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-51041&quot;&gt;&lt;del&gt;SERVER-51041&lt;/del&gt;&lt;/a&gt; Throttle starting transactions for secondary reads&lt;br/&gt;
Branch: v4.2&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/commit/6c3fffb3e82f3208414a747df4fb9ae9ab4e2f52&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/6c3fffb3e82f3208414a747df4fb9ae9ab4e2f52&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="3403698" author="xgen-internal-githook" created="Mon, 21 Sep 2020 18:02:13 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{&apos;name&apos;: &apos;Louis Williams&apos;, &apos;email&apos;: &apos;louis.williams@mongodb.com&apos;, &apos;username&apos;: &apos;louiswilliams&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-51041&quot; title=&quot;Throttle starting transactions for secondary reads&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-51041&quot;&gt;&lt;del&gt;SERVER-51041&lt;/del&gt;&lt;/a&gt; Throttle starting transactions for secondary reads&lt;br/&gt;
Branch: v4.4&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/commit/63a7f9c895de00f6bd440480f943608dfb2670f6&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/63a7f9c895de00f6bd440480f943608dfb2670f6&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="3403053" author="louis.williams" created="Mon, 21 Sep 2020 14:00:56 +0000"  >&lt;p&gt;I don&apos;t believe there are any other areas of immediate concern regarding other operations that use point-in-time reads:&lt;/p&gt;
&lt;ul&gt;
	&lt;li&gt;readConcern &quot;snapshot&quot;: this is designed for multi-document transactions and long-running snapshot reads. By nature, these operations should not be starting new transactions as frequently as local secondary reads&lt;/li&gt;
	&lt;li&gt;readConcern &quot;atClusterTime&quot;: this is used for sharded PIT reads and could also be a victim of the underlying performance issue, but the perf is likely domiated by other bottlenecks in the network and sharding machinery&lt;/li&gt;
	&lt;li&gt;readConcern &quot;linearizable&quot;: very likely dominated by other bottlenecks including waiting for replication&lt;/li&gt;
	&lt;li&gt;readConcern &quot;majority&quot;: see my previous comment&lt;/li&gt;
&lt;/ul&gt;


&lt;p&gt;&#160;&lt;/p&gt;</comment>
                            <comment id="3403034" author="xgen-internal-githook" created="Mon, 21 Sep 2020 13:47:35 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{&apos;name&apos;: &apos;Louis Williams&apos;, &apos;email&apos;: &apos;louis.williams@mongodb.com&apos;, &apos;username&apos;: &apos;louiswilliams&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-51041&quot; title=&quot;Throttle starting transactions for secondary reads&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-51041&quot;&gt;&lt;del&gt;SERVER-51041&lt;/del&gt;&lt;/a&gt; Throttle starting transactions for secondary reads&lt;br/&gt;
Branch: master&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/commit/1f18dc5ce618f61e54d2ac203cbf16b8d388c862&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/1f18dc5ce618f61e54d2ac203cbf16b8d388c862&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="3400862" author="louis.williams" created="Fri, 18 Sep 2020 14:24:09 +0000"  >&lt;p&gt;The reason why we don&apos;t see this issue for majority-committed reads is because &lt;a href=&quot;https://github.com/mongodb/mongo/blob/f79a6251965064176e0a61ae5b48f0f38f3b8b8c/src/mongo/db/storage/wiredtiger/wiredtiger_snapshot_manager.cpp#L84&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;they still take a mutex when starting new transactions&lt;/a&gt;.&lt;/p&gt;</comment>
                    </comments>
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