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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-41186] Benchmark backtrace/unwind techniques</title>
                <link>https://jira.mongodb.org/browse/SERVER-41186</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;It turns out there are 3 semi-reasonable ways to extract a stack on a posix system:&lt;/p&gt;
&lt;ul&gt;
	&lt;li&gt;Assembly extracting sp and winding up&lt;/li&gt;
	&lt;li&gt;Using libunwind&lt;/li&gt;
	&lt;li&gt;Using glibc backtrace&lt;/li&gt;
&lt;/ul&gt;


&lt;p&gt;We need to benchmark these&lt;/p&gt;</description>
                <environment></environment>
        <key id="770016">SERVER-41186</key>
            <summary>Benchmark backtrace/unwind techniques</summary>
                <type id="3" iconUrl="https://jira.mongodb.org/secure/viewavatar?size=xsmall&amp;avatarId=14718&amp;avatarType=issuetype">Task</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="ben.caimano@mongodb.com">Benjamin Caimano</assignee>
                                    <reporter username="ben.caimano@mongodb.com">Benjamin Caimano</reporter>
                        <labels>
                    </labels>
                <created>Thu, 16 May 2019 18:58:39 +0000</created>
                <updated>Wed, 29 May 2019 20:29:23 +0000</updated>
                            <resolved>Wed, 29 May 2019 19:31:15 +0000</resolved>
                                                                                        <votes>0</votes>
                                    <watches>5</watches>
                                                                                                                <comments>
                            <comment id="2264345" author="ben.caimano" created="Wed, 29 May 2019 20:29:23 +0000"  >&lt;p&gt;As an additional note, I tested this with an associated condvar wait after each run. It looks like it was 7us without trace to 12us with. That&apos;s quick enough that it won&apos;t matter at all with waits of any duration, but it will be noticeable if we&apos;re doing trivial waits most of time.&lt;/p&gt;</comment>
                            <comment id="2264248" author="ben.caimano" created="Wed, 29 May 2019 19:30:15 +0000"  >&lt;p&gt;Uploading my mildly hacky benchmarking work here: &lt;span class=&quot;nobr&quot;&gt;&lt;a href=&quot;https://jira.mongodb.org/secure/attachment/218320/218320_backtrace-benchmarks.zip&quot; title=&quot;backtrace-benchmarks.zip attached to SERVER-41186&quot;&gt;backtrace-benchmarks.zip&lt;sup&gt;&lt;img class=&quot;rendericon&quot; src=&quot;https://jira.mongodb.org/images/icons/link_attachment_7.gif&quot; height=&quot;7&quot; width=&quot;7&quot; align=&quot;absmiddle&quot; alt=&quot;&quot; border=&quot;0&quot;/&gt;&lt;/sup&gt;&lt;/a&gt;&lt;/span&gt; .&lt;/p&gt;

&lt;p&gt;My general conclusion is that libunwind backtrace() is almost as fast as working with raw stack unwinding. Posix backtrace() is fairly slower at about 2x. Perhaps unsurprisingly, the libunwind generalized context unwinder is slower than the backtrace function, I suspect it is grabbing and storing a bunch of extra details behind the scenes.&lt;/p&gt;

&lt;p&gt;The big take away is that it&apos;s easily possible for us to take a backtrace ala ~20us, but the call dladdr() takes at least 4x as long. We might want to call dl_iterate_phdr() at start up or scan our elf segments loaded in some other fashion. That way we can translate to offsets from segment when it matters for symbolizing.&lt;/p&gt;</comment>
                            <comment id="2263731" author="ben.caimano" created="Wed, 29 May 2019 15:10:18 +0000"  >&lt;p&gt;This is an investigation ticket, but it still irritates me that it&apos;s not scheduled&lt;/p&gt;</comment>
                    </comments>
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                            <attachment id="218320" name="backtrace-benchmarks.zip" size="15075590" author="ben.caimano@mongodb.com" created="Wed, 29 May 2019 19:23:07 +0000"/>
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