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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-21792] Performance regression with directio enabled on Windows with WiredTiger</title>
                <link>https://jira.mongodb.org/browse/SERVER-21792</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;Issue Status as of Jan 05, 2016&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;br/&gt;
In &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-20991&quot; title=&quot;WiredTiger caching strategy needs improvement on Windows&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-20991&quot;&gt;&lt;del&gt;SERVER-20991&lt;/del&gt;&lt;/a&gt;, &lt;tt&gt;direct_io&lt;/tt&gt; was enabled by default on Windows machines. This change reduced the memory consumption of mongod processes, but may cause a degradation in performance. This change to the default setting of &lt;tt&gt;direct_io&lt;/tt&gt; was reverted in this ticket.&lt;/p&gt;

&lt;p&gt;&lt;b&gt;USER IMPACT&lt;/b&gt;&lt;br/&gt;
With &lt;tt&gt;direct_io&lt;/tt&gt; enabled, when the WiredTiger cache becomes full, the insertion rate may drop significantly.&lt;/p&gt;

&lt;p&gt;&lt;b&gt;WORKAROUNDS&lt;/b&gt;&lt;br/&gt;
To correct the performance regression in MongoDB 3.2.0, start mongod with&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;
		&lt;tbody &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-top: 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;--wiredTigerEngineConfigString=&quot;direct_io=&quot;&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
			&lt;/tbody&gt;
&lt;/table&gt;
&lt;/div&gt;
&lt;p/&gt;

&lt;p&gt;To maintain to the performance characteristics of MongoDB 3.2.0 in later versions, start mongod with&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;
		&lt;tbody &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-top: 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;--wiredTigerEngineConfigString=&quot;direct_io=(data)&quot;&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
			&lt;/tbody&gt;
&lt;/table&gt;
&lt;/div&gt;
&lt;p/&gt;

&lt;p&gt;This setting may also be configured in the mongod configuration file:&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;
		&lt;tbody &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-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;storage:&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: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;    wiredTiger:&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: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;        engineConfig:&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: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;            configString: direct_io=(data)&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
			&lt;/tbody&gt;
&lt;/table&gt;
&lt;/div&gt;
&lt;p/&gt; 

&lt;p&gt;&lt;b&gt;AFFECTED VERSIONS&lt;/b&gt;&lt;br/&gt;
MongoDB 3.2.0&lt;/p&gt;

&lt;p&gt;&lt;b&gt;FIX VERSION&lt;/b&gt;&lt;br/&gt;
The fix is included in the 3.2.1 production release.&lt;/p&gt;
&lt;/div&gt;&lt;/div&gt;

&lt;h6&gt;&lt;a name=&quot;Originaldescription&quot;&gt;&lt;/a&gt;Original description&lt;/h6&gt;

&lt;ul&gt;
	&lt;li&gt;AWS Instance - c4.4xlarge (16 CPU, 30GB RAM)&lt;/li&gt;
	&lt;li&gt;journal and data files configured on separate drives&lt;/li&gt;
	&lt;li&gt;5 threads inserting small records in batches of 1000&lt;/li&gt;
	&lt;li&gt;default cache settings&lt;/li&gt;
	&lt;li&gt;syncdelay 10 (probably not relevant as checkpoints are very long anyway)&lt;/li&gt;
	&lt;li&gt;test run under rc6, same code as ga&lt;/li&gt;
&lt;/ul&gt;


&lt;p&gt;&lt;b&gt;3.0.7&lt;/b&gt;&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/101223/101223_3.0.7.png&quot; width=&quot;800&quot; style=&quot;border: 0px solid black&quot; /&gt;&lt;/span&gt;&lt;/p&gt;

&lt;p&gt;&lt;b&gt;3.2.0&lt;/b&gt;&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/101222/101222_3.2.0-rc6.png&quot; width=&quot;800&quot; style=&quot;border: 0px solid black&quot; /&gt;&lt;/span&gt;&lt;/p&gt;

&lt;ul&gt;
	&lt;li&gt;initially 3.2.0 has somewhat higher insert rate&lt;/li&gt;
	&lt;li&gt;after cache fills 3.0.7 insert rate remains constant but 3.2.0 drops by about 5x, to about 25% of rate under 3.0.7&lt;/li&gt;
&lt;/ul&gt;


&lt;p&gt;Averages insert rates during initial and cache full portions of run:&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;
		&lt;tbody &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-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;            initial   cache full&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;&amp;nbsp;&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: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;3.0.7       152 k/s    135 k/s&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: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;3.2.0       178 k/s     33 k/s&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
			&lt;/tbody&gt;
&lt;/table&gt;
&lt;/div&gt;
&lt;p/&gt;</description>
                <environment></environment>
        <key id="242715">SERVER-21792</key>
            <summary>Performance regression with directio enabled on Windows with WiredTiger</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="mark.benvenuto@mongodb.com">Mark Benvenuto</assignee>
                                    <reporter username="bruce.lucas@mongodb.com">Bruce Lucas</reporter>
                        <labels>
                            <label>code-only</label>
                    </labels>
                <created>Tue, 8 Dec 2015 12:47:16 +0000</created>
                <updated>Mon, 8 Jan 2024 15:23:03 +0000</updated>
                            <resolved>Tue, 15 Dec 2015 23:59:59 +0000</resolved>
                                    <version>3.2.0</version>
                                    <fixVersion>3.2.1</fixVersion>
                    <fixVersion>3.3.0</fixVersion>
                                    <component>WiredTiger</component>
                                        <votes>0</votes>
                                    <watches>20</watches>
                                                                                                                <comments>
                            <comment id="1147352" author="mark.benvenuto" created="Tue, 19 Jan 2016 16:03:32 +0000"  >&lt;p&gt;The Yaml configuration setting is &lt;a href=&quot;https://github.com/mongodb/mongo/blob/master/src/mongo/db/storage/wiredtiger/wiredtiger_global_options.cpp#L70&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;hidden&lt;/a&gt;:&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;
		&lt;tbody &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-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;storage:&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: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;    wiredTiger:&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: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;        engineConfig:&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: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;            configString: direct_io=(data)&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
			&lt;/tbody&gt;
&lt;/table&gt;
&lt;/div&gt;
&lt;p/&gt;</comment>
                            <comment id="1146362" author="michael@aorato.com" created="Mon, 18 Jan 2016 12:11:08 +0000"  >&lt;p&gt;How can we configure this setting: wiredTigerEngineConfigString=&quot;direct_io=(data)&quot; in the cfg file?&lt;/p&gt;</comment>
                            <comment id="1121011" author="nick@innsenroute.com" created="Tue, 22 Dec 2015 23:06:06 +0000"  >&lt;p&gt;@Michael Dolinsky - the original fix (which this reverts) was to fix that exact problem. Running with direct_io gives poor query performance, running without direct_io will cause all the system memory to be consumed. In 3.2, this wasn&apos;t the case (for my workload). I had good query performance and constant memory usage. I didn&apos;t have any regressions as described above.&lt;/p&gt;

&lt;p&gt;Now it sounds like the caching strategy for windows may be revisited for the next major release.&lt;/p&gt;</comment>
                            <comment id="1118987" author="michael@aorato.com" created="Sun, 20 Dec 2015 16:54:07 +0000"  >&lt;p&gt;So with 3.2.1 if there are a lot of writes (more then the system can handle) essentially the system cache will take up all memory and would cause an OS issue / MongoDB issue&lt;br/&gt;
Have you run this stress test to see this for yourself?&lt;/p&gt;</comment>
                            <comment id="1117020" author="dan@10gen.com" created="Thu, 17 Dec 2015 14:07:20 +0000"  >&lt;p&gt;The nightly development releases can be found here: &lt;br/&gt;
&lt;a href=&quot;https://www.mongodb.org/downloads/#development&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://www.mongodb.org/downloads/#development&lt;/a&gt;&lt;/p&gt;

&lt;p&gt;From there, you have to select your os version and mongodb version&lt;/p&gt;</comment>
                            <comment id="1116661" author="michael@aorato.com" created="Wed, 16 Dec 2015 23:48:14 +0000"  >&lt;p&gt;Where can we download the 3.2.1 bits to test this?&lt;/p&gt;</comment>
                            <comment id="1115628" author="mark.benvenuto" created="Wed, 16 Dec 2015 00:58:35 +0000"  >&lt;p&gt;In 3.2.1, the old behavior which reduces memory usage, but has a performance regression in some scenarios can be achieved as followed: &lt;tt&gt;--wiredTigerEngineConfigString=&quot;direct_io=(data)&quot;&lt;/tt&gt;.&lt;/p&gt;</comment>
                            <comment id="1115568" author="mark.benvenuto" created="Tue, 15 Dec 2015 23:59:40 +0000"  >&lt;p&gt;Master: &lt;a href=&quot;https://github.com/mongodb/mongo/commit/439a56d7af3ce4bad983f5829b3485bb0af7f6c3&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/439a56d7af3ce4bad983f5829b3485bb0af7f6c3&lt;/a&gt;&lt;/p&gt;

&lt;p&gt;v3.2: &lt;a href=&quot;https://github.com/mongodb/mongo/commit/c7b065227470a27c40c45f07a8c967b7aa7af9db&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/c7b065227470a27c40c45f07a8c967b7aa7af9db&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="1110612" author="ramon.fernandez" created="Thu, 10 Dec 2015 14:03:00 +0000"  >&lt;p&gt;&lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=stephen.jannin%40sgcib.com&quot; class=&quot;user-hover&quot; rel=&quot;stephen.jannin@sgcib.com&quot;&gt;stephen.jannin@sgcib.com&lt;/a&gt;, your statement is incorrect: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-19795&quot; title=&quot;mongod memory consumption higher than WT cache size (at least on Windows 2008 R2)&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-19795&quot;&gt;&lt;del&gt;SERVER-19795&lt;/del&gt;&lt;/a&gt; is not a memory leak. As per &lt;a href=&quot;https://docs.mongodb.org/v3.0/faq/storage/#to-what-size-should-i-set-the-wiredtiger-cache&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;the documentation&lt;/a&gt;:&lt;/p&gt;
&lt;blockquote&gt;
&lt;p&gt;The storage.wiredTiger.engineConfig.cacheSizeGB only limits the size of the WiredTiger cache, not the total amount of memory used by mongod. The WiredTiger cache is only one component of the RAM used by MongoDB. MongoDB also automatically uses all free memory on the machine via the filesystem cache (data in the filesystem cache is compressed).&lt;/p&gt;

&lt;p&gt;In addition, the operating system will use any free RAM to buffer filesystem blocks.&lt;/p&gt;

&lt;p&gt;To accommodate the additional consumers of RAM, you may have to decrease WiredTiger cache size. Avoid increasing the WiredTiger cache size above its default value.&lt;/p&gt;&lt;/blockquote&gt;</comment>
                            <comment id="1110448" author="stephen.jannin@sgcib.com" created="Thu, 10 Dec 2015 08:05:26 +0000"  >&lt;p&gt;Warning, if you use direct_io, you have very poor performances on windows.&lt;br/&gt;
If you don&apos;t use direct_io, you have a severe memory leak that has never been fixed. See &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-19795&quot; class=&quot;external-link&quot; rel=&quot;nofollow&quot;&gt;https://jira.mongodb.org/browse/SERVER-19795&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="1109532" author="bruce.lucas@10gen.com" created="Wed, 9 Dec 2015 16:18:08 +0000"  >&lt;p&gt;A run with Windows performance counters, with default settings (direct i/o enabled). &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/101429/101429_3.2.0-rc6-perf.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;performance drops at A&lt;/li&gt;
	&lt;li&gt;coinciding with this we see memory starting to be paged out&lt;/li&gt;
	&lt;li&gt;but, as expected, system &quot;cache bytes&quot; is small because mongod is doing direct i/o instead of going through the system file cache, and available memory is still about half - so why the paging activity?&lt;/li&gt;
	&lt;li&gt;no significant paging in, presumably because we don&apos;t actually need the stuff that&apos;s been paged out or because it hasn&apos;t actually been evicted from memory (since there&apos;s no memory pressure)&lt;/li&gt;
&lt;/ul&gt;


&lt;p&gt;Theory: when memory usage reaches a certain point, the o/s starts spilling memory to disk proactively (not in response to memory pressure as such), and for some reason tbd this in itself has a negative performance impact. Note that the following do not appear to be the reason for the negative performance impact:&lt;/p&gt;
&lt;ul&gt;
	&lt;li&gt;having to page in the memory that has been paged out - we don&apos;t see page read activity&lt;/li&gt;
	&lt;li&gt;i/o contention at the volume level with mongod writes - pagefile, data, and journal are all on separately provisioned volumes.&lt;/li&gt;
&lt;/ul&gt;


&lt;p&gt;Why do we not see this effect with direct i/o disabled? We&apos;re doing a run now with direct i/o disabled to see how the stats differ. Theory: as long as there is dirty file data in the file cache Windows preferentially writes that instead of paging process memory out, so by in effect disabling the file cache we&apos;ve given Windows the green light to page process memory out, and that for reasons tbd is having a negative performance impact.&lt;/p&gt;</comment>
                            <comment id="1109500" author="xgen-internal-githook" created="Wed, 9 Dec 2015 16:02:03 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{u&apos;name&apos;: u&apos;Ramon Fernandez&apos;, u&apos;email&apos;: u&apos;ramon@mongodb.com&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/DOCS-6758&quot; title=&quot;Add SERVER-21792 to 3.2 errata&quot; class=&quot;issue-link&quot; data-issue-key=&quot;DOCS-6758&quot;&gt;&lt;del&gt;DOCS-6758&lt;/del&gt;&lt;/a&gt; Add &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-21792&quot; title=&quot;Performance regression with directio enabled on Windows with WiredTiger&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-21792&quot;&gt;&lt;del&gt;SERVER-21792&lt;/del&gt;&lt;/a&gt; to errata&lt;/p&gt;

&lt;p&gt;Signed-off-by: kay &amp;lt;kay.kim@10gen.com&amp;gt;&lt;br/&gt;
Branch: master&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/docs/commit/a3b4bee303ff56480524fb23bed37e93949e5f66&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/docs/commit/a3b4bee303ff56480524fb23bed37e93949e5f66&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="1108774" author="bruce.lucas@10gen.com" created="Tue, 8 Dec 2015 21:39:38 +0000"  >&lt;p&gt;This seems to be a result using direct i/o, which was enabled for Windows in rc3 under &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-20991&quot; title=&quot;WiredTiger caching strategy needs improvement on Windows&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-20991&quot;&gt;&lt;del&gt;SERVER-20991&lt;/del&gt;&lt;/a&gt;; here&apos;s a comparable run with direct i/o disabled. Initial throughput is the same as or better than initial throughput with direct i/o enabled, and remains consistent throughout the run.&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/101316/101316_3.2.0-rc6-nodirectio.png&quot; width=&quot;800&quot; style=&quot;border: 0px solid black&quot; /&gt;&lt;/span&gt;&lt;/p&gt;

&lt;p&gt;Adding this result to the table above:&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;
		&lt;tbody &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-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;                       initial   cache full&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;&amp;nbsp;&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: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;3.0.7                  152 k/s    135 k/s&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: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;3.2.0, default         178 k/s     33 k/s&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: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;3.2.0, no direct i/o   186 k/s    187 k/s&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
			&lt;/tbody&gt;
&lt;/table&gt;
&lt;/div&gt;
&lt;p/&gt;

&lt;p&gt;Direct i/o can be disabled by specifying --wiredTigerEngineConfigString &quot;direct_io=()&quot; (or equivalent config file option), and this should be a useful workaround for now.&lt;/p&gt;</comment>
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