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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-16881] Pause due to sudden 4GB increase in cached bytes under WiredTiger</title>
                <link>https://jira.mongodb.org/browse/SERVER-16881</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;During one 10-minute run of a heavy mixed workload a period of several seconds of very low throughput was seen:&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/61880/61880_4GB-jump.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;During the pause all statistics appear to be consistent with a large number of cache evictions, resulting in a lot of writes, system cpu time, and context switches. This heavy eviction load seems like a plausible cause for the low throughput during this period.&lt;/li&gt;
	&lt;li&gt;Immediately before the pause the number of bytes in the cache jumped by about 4 GiB between two samples about 0.1 seconds apart. This is a plausible trigger for the heavy eviction load.&lt;/li&gt;
	&lt;li&gt;During the pause bytes and pages in cache declined slowly, presumably a result of the page evictions.&lt;/li&gt;
	&lt;li&gt;The pause ends with a sudden drop in the size of the cache by about 4 GiB between two samples about 0.1 seconds apart.&lt;/li&gt;
&lt;/ul&gt;


&lt;p&gt;Here are the specific samples&lt;/p&gt;

&lt;ul&gt;
	&lt;li&gt;just before increase&lt;br/&gt;
&quot;localTime&quot;:&quot;2015-01-15T18:42:20.344Z&quot;&lt;br/&gt;
&quot;bytes currently in the cache&quot;:4315898330&lt;br/&gt;
&quot;pages currently held in the cache&quot;:60756&lt;/li&gt;
&lt;/ul&gt;


&lt;ul&gt;
	&lt;li&gt;just after increase&lt;br/&gt;
&quot;localTime&quot;:&quot;2015-01-15T18:42:20.472Z&quot;&lt;br/&gt;
&quot;bytes currently in the cache&quot;:8594367123&lt;br/&gt;
&quot;pages currently held in the cache&quot;:60598&lt;/li&gt;
&lt;/ul&gt;


&lt;ul&gt;
	&lt;li&gt;just before decrease&lt;br/&gt;
&quot;localTime&quot;:&quot;2015-01-15T18:42:25.058Z&quot;&lt;br/&gt;
&quot;bytes currently in the cache&quot;:7242022268&lt;br/&gt;
&quot;pages currently held in the cache&quot;:35169&lt;/li&gt;
&lt;/ul&gt;


&lt;ul&gt;
	&lt;li&gt;just after decrease&lt;br/&gt;
&quot;localTime&quot;:&quot;2015-01-15T18:42:25.170Z&quot;&lt;br/&gt;
&quot;bytes currently in the cache&quot;:2941126278&lt;br/&gt;
&quot;pages currently held in the cache&quot;:35137&lt;/li&gt;
&lt;/ul&gt;


&lt;p&gt;The size of the sudden jump was: 7242022268 - 2941126278 = 3.98 GiB&lt;/p&gt;

&lt;p&gt;The size of the sudden drop was: 8594367123 - 4315898330 = 4.01 GiB&lt;/p&gt;

&lt;p&gt;Neither a sudden increase nor a sudden decrease of cache size by 4 GiB in 0.1 seconds due to real cached data is plausible.&lt;/p&gt;</description>
                <environment></environment>
        <key id="179042">SERVER-16881</key>
            <summary>Pause due to sudden 4GB increase in cached bytes under 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="keith.bostic@mongodb.com">Keith Bostic</assignee>
                                    <reporter username="bruce.lucas@mongodb.com">Bruce Lucas</reporter>
                        <labels>
                            <label>wiredtiger</label>
                    </labels>
                <created>Thu, 15 Jan 2015 21:20:21 +0000</created>
                <updated>Mon, 26 Jan 2015 15:35:53 +0000</updated>
                            <resolved>Wed, 21 Jan 2015 20:06:57 +0000</resolved>
                                    <version>2.8.0-rc5</version>
                                    <fixVersion>3.0.0-rc6</fixVersion>
                                    <component>Storage</component>
                                        <votes>0</votes>
                                    <watches>13</watches>
                                                                                                                <comments>
                            <comment id="813046" author="keith.bostic" created="Mon, 26 Jan 2015 15:35:53 +0000"  >&lt;p&gt;This issue was caused by an accounting bug in tracking the memory currently held in cache, and I recall &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-16902&quot; class=&quot;external-link&quot; rel=&quot;nofollow&quot;&gt;SERVER-16902&lt;/a&gt; was a problem in selecting pages for splitting. (The accounting problem certainly might have exacerbated16902, though.)&lt;/p&gt;</comment>
                            <comment id="812999" author="mdcallag" created="Mon, 26 Jan 2015 14:43:09 +0000"  >&lt;p&gt;Is this caused by the intermittent use of huge, as in 4GB large), leaf pages? See comments from &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-16902&quot; class=&quot;external-link&quot; rel=&quot;nofollow&quot;&gt;https://jira.mongodb.org/browse/SERVER-16902&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="808968" author="keith.bostic" created="Tue, 20 Jan 2015 23:03:51 +0000"  >&lt;p&gt;Thanks, &lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=bruce.lucas%4010gen.com&quot; class=&quot;user-hover&quot; rel=&quot;bruce.lucas@10gen.com&quot;&gt;bruce.lucas@10gen.com&lt;/a&gt;.&lt;/p&gt;

&lt;p&gt;If I instrument a WiredTiger branch to drop core when this happens, and build a binary on MCI, would that make it possible for you to give me a stack trace? (Not sure if MCI builds objects with symbol tables?)&lt;/p&gt;</comment>
                    </comments>
                    <attachments>
                            <attachment id="61880" name="4GB-jump.png" size="306954" author="bruce.lucas@mongodb.com" created="Thu, 15 Jan 2015 21:20:21 +0000"/>
                            <attachment id="62292" name="9cf04338af2dd4f9ac46e5d74345c7ea228d9e8f.png" size="56637" author="bruce.lucas@mongodb.com" created="Tue, 20 Jan 2015 22:53:52 +0000"/>
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