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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-28169] Set eviction=(threads_min=1) in Microbenchmarks testing</title>
                <link>https://jira.mongodb.org/browse/SERVER-28169</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;&lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-28026&quot; title=&quot;Disable auto-tuning of WiredTiger eviction threads&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-28026&quot;&gt;&lt;del&gt;SERVER-28026&lt;/del&gt;&lt;/a&gt; changed a wiredTiger / inMemory internal setting&lt;/p&gt;

&lt;p/&gt;
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&lt;p/&gt;

&lt;p&gt;...from 0 to 4. This seems to interfere with results for the Microbenchmarks suite and in any case we generally turn off such background processes in Microbenchmarks. I will set this back to zero using &lt;tt&gt;--inMemoryEngineConfigString&lt;/tt&gt;.&lt;/p&gt;</description>
                <environment></environment>
        <key id="360000">SERVER-28169</key>
            <summary>Set eviction=(threads_min=1) in Microbenchmarks testing</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="henrik.ingo@mongodb.com">Henrik Ingo</assignee>
                                    <reporter username="henrik.ingo@mongodb.com">Henrik Ingo</reporter>
                        <labels>
                    </labels>
                <created>Thu, 2 Mar 2017 13:56:59 +0000</created>
                <updated>Fri, 31 Mar 2017 18:44:29 +0000</updated>
                            <resolved>Mon, 6 Mar 2017 21:39:06 +0000</resolved>
                                                    <fixVersion>3.2.13</fixVersion>
                    <fixVersion>3.4.4</fixVersion>
                    <fixVersion>3.5.4</fixVersion>
                                    <component>Performance</component>
                                        <votes>0</votes>
                                    <watches>4</watches>
                                                                                                                <comments>
                            <comment id="1537758" author="xgen-internal-githook" created="Fri, 31 Mar 2017 11:05:01 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{u&apos;username&apos;: u&apos;henrikingo&apos;, u&apos;name&apos;: u&apos;Henrik Ingo&apos;, u&apos;email&apos;: u&apos;henrik.ingo@mongodb.com&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-28169&quot; title=&quot;Set eviction=(threads_min=1) in Microbenchmarks testing&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-28169&quot;&gt;&lt;del&gt;SERVER-28169&lt;/del&gt;&lt;/a&gt; Set eviction=(threads_min=1) in Microbenchmarks testing&lt;/p&gt;

&lt;p&gt;Undoing the effect of 5a3ecdc731673c82cc841eaecf2bc59067c067be for microbenchmarks.&lt;/p&gt;

&lt;p&gt;(cherry picked from commit b078623660b1f4582cbb90998f8e6c252d3bd7b8)&lt;br/&gt;
Branch: v3.4&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/commit/4a3ecd3dd5ff54b478eb7d4b5c28114976c6995b&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/4a3ecd3dd5ff54b478eb7d4b5c28114976c6995b&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="1532036" author="xgen-internal-githook" created="Fri, 24 Mar 2017 13:53:00 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{u&apos;username&apos;: u&apos;henrikingo&apos;, u&apos;name&apos;: u&apos;Henrik Ingo&apos;, u&apos;email&apos;: u&apos;henrik.ingo@mongodb.com&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-28169&quot; title=&quot;Set eviction=(threads_min=1) in Microbenchmarks testing&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-28169&quot;&gt;&lt;del&gt;SERVER-28169&lt;/del&gt;&lt;/a&gt; Set eviction=(threads_min=1) in Microbenchmarks testing&lt;/p&gt;

&lt;p&gt;Undoing the effect of 5a3ecdc731673c82cc841eaecf2bc59067c067be for microbenchmarks.&lt;/p&gt;

&lt;p&gt;(cherry picked from commit b078623660b1f4582cbb90998f8e6c252d3bd7b8)&lt;br/&gt;
Branch: v3.2&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/commit/ca880118d629ceeec952cc526ec71ddc08aea739&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/ca880118d629ceeec952cc526ec71ddc08aea739&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="1517249" author="henrik.ingo@10gen.com" created="Mon, 6 Mar 2017 21:38:18 +0000"  >&lt;p&gt;Master done. Needs backporting once 3.4 is unfrozen.&lt;/p&gt;</comment>
                            <comment id="1517220" author="xgen-internal-githook" created="Mon, 6 Mar 2017 21:18:41 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{u&apos;username&apos;: u&apos;henrikingo&apos;, u&apos;name&apos;: u&apos;Henrik Ingo&apos;, u&apos;email&apos;: u&apos;henrik.ingo@mongodb.com&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-28169&quot; title=&quot;Set eviction=(threads_min=1) in Microbenchmarks testing&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-28169&quot;&gt;&lt;del&gt;SERVER-28169&lt;/del&gt;&lt;/a&gt; Set eviction=(threads_min=1) in Microbenchmarks testing&lt;/p&gt;

&lt;p&gt;Undoing the effect of 5a3ecdc731673c82cc841eaecf2bc59067c067be for microbenchmarks.&lt;br/&gt;
Branch: master&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/commit/b078623660b1f4582cbb90998f8e6c252d3bd7b8&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/b078623660b1f4582cbb90998f8e6c252d3bd7b8&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="1515197" author="henrik.ingo@10gen.com" created="Fri, 3 Mar 2017 11:23:27 +0000"  >&lt;p&gt;Thanks &lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=michael.cahill&quot; class=&quot;user-hover&quot; rel=&quot;michael.cahill&quot;&gt;michael.cahill&lt;/a&gt;&lt;/p&gt;

&lt;p&gt;I realized this, but my first instinct is to just try to revert the recent change back to what it was.&lt;/p&gt;

&lt;p&gt;It&apos;s a good rule in performance testing to only do one change at a time, and for now the focus is simply to identify what has changed in our results, around Feb 21 or so, and how can we get back to where we were before. A followup action could indeed be to set threads_max lower as well.&lt;/p&gt;

&lt;p&gt;Second, while I understand how the new default of threads_min would in general contribute to more stable performance behavior, I&apos;m suspecting the microbenchmarks behave differently from real world workloads here. They&apos;re mostly just a few hundred rows or so, and in this case we&apos;re talking readonly tests. So the theory is that just getting back to how things were, will in fact get rid of the occasional drops we&apos;re now seeing. If we first observe that (for some weeks), then we could indeed tae a further step of locking both min_threads and max_threads at 2, or even 1. (In microbenchmarks we generally just turn off everything, but it might not be feasible here to use 0. As you remember, we don&apos;t restart mongod between tests, so some eviction activity might currently be necessary.)&lt;/p&gt;</comment>
                            <comment id="1515092" author="michael.cahill" created="Fri, 3 Mar 2017 06:14:39 +0000"  >&lt;p&gt;I think it&apos;s worth clarifying what this setting controls.&lt;/p&gt;

&lt;p&gt;The WiredTiger and inMemory storage engines have always had the setting &lt;tt&gt;eviction=(threads_max=4)&lt;/tt&gt;.  That means when the cache becomes full and eviction starts running, up to 4 background threads will be started.&lt;/p&gt;

&lt;p&gt;As &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-28026&quot; title=&quot;Disable auto-tuning of WiredTiger eviction threads&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-28026&quot;&gt;&lt;del&gt;SERVER-28026&lt;/del&gt;&lt;/a&gt; explains, some WiredTiger work in &lt;a href=&quot;https://jira.mongodb.org/browse/WT-2898&quot; title=&quot;Improve performance of eviction-heavy workloads by dynamically controlling the number of eviction threads&quot; class=&quot;issue-link&quot; data-issue-key=&quot;WT-2898&quot;&gt;&lt;del&gt;WT-2898&lt;/del&gt;&lt;/a&gt; changes when those background threads are started.  We saw &lt;b&gt;higher&lt;/b&gt; variation in test runs without that configuration change because sometimes background threads would start and sometimes they wouldn&apos;t.  That is what led to changing the defaults to &lt;tt&gt;eviction=(threads_min=4,threads_max=4)&lt;/tt&gt; &amp;#8211; i.e., always start 4 eviction threads.&lt;/p&gt;

&lt;p&gt;I understand wanting to minimize the impact of background threads on the microbenchmarks, but I suggest that if less threads improves the test results, we try something like &lt;tt&gt;eviction=(threads_min=2,threads_max=2)&lt;/tt&gt;.  That is, keep the min/max settings equal but make them both smaller.&lt;/p&gt;</comment>
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