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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-12157] Replace implementation of StringMapDefaultHash::operator() with Murmur3.</title>
                <link>https://jira.mongodb.org/browse/SERVER-12157</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;The existing hash function used for StringMap has a very high collision rate for text names that vary by only a few characters; particularly suffixes.  Running the Smhasher benchmark&apos;s text and performance tests shows that Murmur3 is superior in terms of collisions and comparable in terms of performance for short keys (and superior for long keys).&lt;/p&gt;

&lt;p&gt;So, the task of this ticket is to replace the current implementation with Murmur3.  I recommend doing this by making StringDataDefaultHash a typedef of StringData::Hasher, and changing the implementation of SD::Hasher::operator() as follows:&lt;/p&gt;

&lt;p&gt;On platforms where size_t is 32-bit, we should use MurmurHash3_x86_32.  Where it is 64 bits, we should use MurmurHash3_x64_128 and keep the low order 64 bits.&lt;/p&gt;</description>
                <environment></environment>
        <key id="103128">SERVER-12157</key>
            <summary>Replace implementation of StringMapDefaultHash::operator() with Murmur3.</summary>
                <type id="4" iconUrl="https://jira.mongodb.org/secure/viewavatar?size=xsmall&amp;avatarId=14710&amp;avatarType=issuetype">Improvement</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="davide.italiano">Davide Italiano</assignee>
                                    <reporter username="schwerin@mongodb.com">Andy Schwerin</reporter>
                        <labels>
                    </labels>
                <created>Wed, 18 Dec 2013 20:05:31 +0000</created>
                <updated>Mon, 11 Jul 2016 17:37:41 +0000</updated>
                            <resolved>Sat, 4 Jan 2014 00:02:41 +0000</resolved>
                                    <version>2.5.4</version>
                                    <fixVersion>2.5.5</fixVersion>
                                    <component>Internal Code</component>
                    <component>Performance</component>
                                        <votes>0</votes>
                                    <watches>4</watches>
                                                                                                                <comments>
                            <comment id="477337" author="xgen-internal-githook" created="Sun, 5 Jan 2014 10:15:51 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{u&apos;username&apos;: u&apos;dcci&apos;, u&apos;name&apos;: u&apos;Davide Italiano&apos;, u&apos;email&apos;: u&apos;davide.italiano@10gen.com&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-12157&quot; title=&quot;Replace implementation of StringMapDefaultHash::operator() with Murmur3.&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-12157&quot;&gt;&lt;del&gt;SERVER-12157&lt;/del&gt;&lt;/a&gt;: fix build on Linux 32-bit and old version(s) of g++.&lt;br/&gt;
Branch: master&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/commit/4cd551544a4f25dcb5c2447b9ee14cf0591a3cfe&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/4cd551544a4f25dcb5c2447b9ee14cf0591a3cfe&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="477207" author="xgen-internal-githook" created="Sat, 4 Jan 2014 16:25:36 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{u&apos;username&apos;: u&apos;erh&apos;, u&apos;name&apos;: u&apos;Eliot Horowitz&apos;, u&apos;email&apos;: u&apos;eliot@10gen.com&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-12157&quot; title=&quot;Replace implementation of StringMapDefaultHash::operator() with Murmur3.&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-12157&quot;&gt;&lt;del&gt;SERVER-12157&lt;/del&gt;&lt;/a&gt;: fix 32-bit compile&lt;br/&gt;
Branch: master&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/commit/06851f4859a61019a17147ea96c4e6d9462f354b&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/06851f4859a61019a17147ea96c4e6d9462f354b&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="477107" author="xgen-internal-githook" created="Sat, 4 Jan 2014 00:01:46 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{u&apos;username&apos;: u&apos;dcci&apos;, u&apos;name&apos;: u&apos;Davide Italiano&apos;, u&apos;email&apos;: u&apos;davide.italiano@10gen.com&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-12157&quot; title=&quot;Replace implementation of StringMapDefaultHash::operator() with Murmur3.&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-12157&quot;&gt;&lt;del&gt;SERVER-12157&lt;/del&gt;&lt;/a&gt; Replace implementation of StringMapDefaultHash::operator()&lt;br/&gt;
with Murmur3. This has proven to be more effective in terms of speed&lt;br/&gt;
and collision rates.&lt;br/&gt;
Branch: master&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/commit/22424987d2f0df32671998f0c7574adcfa07bf32&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/22424987d2f0df32671998f0c7574adcfa07bf32&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="472719" author="schwerin" created="Wed, 18 Dec 2013 20:09:08 +0000"  >&lt;p&gt;FYI, MurmurHash3 functions are available by including &quot;murmurhash3/MurmurHash3.h&quot;.&lt;/p&gt;</comment>
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