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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>
    </build-info>


<item>
            <title>[SERVER-16662] Extended pauses in WiredTiger when workers greatly exceeds number of cores</title>
                <link>https://jira.mongodb.org/browse/SERVER-16662</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;This is a continuation of &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-16269&quot; title=&quot;WiredTiger blocks queries and updates for many seconds.&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-16269&quot;&gt;&lt;del&gt;SERVER-16269&lt;/del&gt;&lt;/a&gt;.&lt;/p&gt;
&lt;ul&gt;
	&lt;li&gt;tests run on Ubuntu, VMWare, 6 cores.&lt;/li&gt;
	&lt;li&gt;THP confirmed disabled (both /sys/kernel/mm/transparent_hugepage/{defrag,enabled} set to &quot;never&quot;).&lt;/li&gt;
	&lt;li&gt;using a recent pre-rc4 build from master (6105f06402fe1e7578d41f3e4e583a1476ef2455).&lt;/li&gt;
&lt;/ul&gt;


&lt;p&gt;Under heavy mixed workload with 100 clients, periodic extended pauses of several seconds with zero throughput are seen, for example at A and B:&lt;/p&gt;

&lt;p&gt;&lt;span class=&quot;image-wrap&quot; style=&quot;&quot;&gt;&lt;img src=&quot;https://jira.mongodb.org/secure/attachment/60422/60422_wt-pre4-jp-100.png&quot; width=&quot;900&quot; style=&quot;border: 0px solid black&quot; /&gt;&lt;/span&gt;&lt;/p&gt;

&lt;ul&gt;
	&lt;li&gt;Each pause ends with reports in mongod log of operations taking as long as the pause, up to 11852 ms in this test (you can read this off of the &quot;max logged query&quot; graph: vertical axis is longest operation, in ms, logged in each 0.1 second interval). The application would see this as an anomaly.&lt;/li&gt;
	&lt;li&gt;During each pause system cpu utilization and context switch rate goes very high. This is consistent with multiple threads executing a spin lock calling sched_yield. The high CPU contention due to the spin lock and a high ratio of threads to cores would extend the pause by starving the thread it is waiting for.&lt;/li&gt;
	&lt;li&gt;There is a general roughness, i.e. very short spikes, on the order of the sampling interval of 0.1 seconds, in system CPU and context switches. These seem to correlate with a similar roughness, i.e. very short dips, in the overall throughput. This suggests that even outside the pauses overall throughput is being affected by CPU contention due to the sched_yield spin locks.&lt;/li&gt;
	&lt;li&gt;The particularly long pause at B seems to be associated with the end of a checkpoint.&lt;/li&gt;
&lt;/ul&gt;


&lt;p&gt;The length of the pauses is roughly proportional to the number of threads, supporting the theory that the CPU contention due to the spin lock exacerbates the pauses. Here&apos;s the same 200 second run with 50 clients, showing pauses roughly one half the length of pauses with 100 clients above.&lt;/p&gt;

&lt;p&gt;&lt;span class=&quot;image-wrap&quot; style=&quot;&quot;&gt;&lt;img src=&quot;https://jira.mongodb.org/secure/attachment/60421/60421_wt-pre4-jp-50.png&quot; width=&quot;900&quot; style=&quot;border: 0px solid black&quot; /&gt;&lt;/span&gt;&lt;/p&gt;

&lt;p&gt;Similar symptoms are seen in a less extreme form with a 100-thread insert-only workload:&lt;/p&gt;

&lt;p&gt;&lt;span class=&quot;image-wrap&quot; style=&quot;&quot;&gt;&lt;img src=&quot;https://jira.mongodb.org/secure/attachment/60420/60420_try15.png&quot; width=&quot;900&quot; style=&quot;border: 0px solid black&quot; /&gt;&lt;/span&gt;&lt;/p&gt;

&lt;ul&gt;
	&lt;li&gt;The periodic dips (A, B, C, D) are briefer in this insert-only workload than in the mixed workload above. This suggests that non-insert operations, probably reads, are particularly affected by the periodic dips.&lt;/li&gt;
	&lt;li&gt;The disruption from E to F, apparently associated with the end of a checkpoint, is as bad for the insert-only workload as it is for the mixed workload: mongod reports operations taking up to 11305 ms. This suggests that this pause is may be a little different as it appears to affect inserts as well.&lt;/li&gt;
&lt;/ul&gt;


&lt;p&gt;Insert-only workload:&lt;/p&gt;
&lt;p/&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: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;    every = 1000&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;    var t = new Date()&lt;/span&gt;&lt;/pre&gt;
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				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&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;
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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;   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;insert(200000)&lt;/span&gt;&lt;/pre&gt;
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			&lt;/tbody&gt;
&lt;/table&gt;
&lt;/div&gt;
&lt;p/&gt;</description>
                <environment></environment>
        <key id="176061">SERVER-16662</key>
            <summary>Extended pauses in WiredTiger when workers greatly exceeds number of cores</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="david.hows">David Hows</assignee>
                                    <reporter username="bruce.lucas@mongodb.com">Bruce Lucas</reporter>
                        <labels>
                            <label>wiredtiger</label>
                    </labels>
                <created>Wed, 24 Dec 2014 13:57:14 +0000</created>
                <updated>Fri, 24 Apr 2015 05:14:16 +0000</updated>
                            <resolved>Mon, 23 Mar 2015 19:36:44 +0000</resolved>
                                                    <fixVersion>3.0.0</fixVersion>
                                    <component>Storage</component>
                    <component>WiredTiger</component>
                                        <votes>0</votes>
                                    <watches>26</watches>
                                                                                                                <comments>
                            <comment id="803802" author="michael.cahill" created="Tue, 13 Jan 2015 22:26:10 +0000"  >&lt;p&gt;This workload should be considerably improved by RC5.&lt;/p&gt;</comment>
                            <comment id="800958" author="john.page" created="Fri, 9 Jan 2015 10:54:52 +0000"  >&lt;p&gt;Just seen/realised this is especially pertinent in a sharded setup as each monogs router will make a whole bunch of connections and thus threads - I&apos;m seeing as I add routers - overall performance drops in 2.8  - However! it also does with mmapv1 and it doesn&apos;t in 2.6 so this is more than simply wired tiger related.&lt;/p&gt;</comment>
                    </comments>
                <issuelinks>
                            <issuelinktype id="10011">
                    <name>Depends</name>
                                                                <inwardlinks description="is depended on by">
                                        <issuelink>
            <issuekey id="200036">WT-1901</issuekey>
        </issuelink>
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                            <issuelinktype id="10012">
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                                        <issuelink>
            <issuekey id="182835">SERVER-17194</issuekey>
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                                                        </issuelinktype>
                            <issuelinktype id="10220">
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                                                        </outwardlinks>
                                                        </issuelinktype>
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                            <attachment id="60420" name="try15.png" size="172673" author="bruce.lucas@mongodb.com" created="Wed, 24 Dec 2014 13:57:14 +0000"/>
                            <attachment id="60466" name="wt-pre4-jp-100-nolog.png" size="173815" author="bruce.lucas@mongodb.com" created="Fri, 26 Dec 2014 13:36:58 +0000"/>
                            <attachment id="60422" name="wt-pre4-jp-100.png" size="163016" author="bruce.lucas@mongodb.com" created="Wed, 24 Dec 2014 13:57:14 +0000"/>
                            <attachment id="60421" name="wt-pre4-jp-50.png" size="161928" author="bruce.lucas@mongodb.com" created="Wed, 24 Dec 2014 13:57:14 +0000"/>
                            <attachment id="60684" name="wt-rc4-jp-50-nojournal-pause1.png" size="464434" author="bruce.lucas@mongodb.com" created="Wed, 31 Dec 2014 21:22:22 +0000"/>
                            <attachment id="60685" name="wt-rc4-jp-50-nojournal-pause2.png" size="459466" author="bruce.lucas@mongodb.com" created="Wed, 31 Dec 2014 21:22:22 +0000"/>
                            <attachment id="60686" name="wt-rc4-jp-50-nojournal.png" size="155983" author="bruce.lucas@mongodb.com" created="Wed, 31 Dec 2014 21:22:22 +0000"/>
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