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annotate xml/en/docs/hash.xml @ 3030:cd1d5312e47c

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Updated with Netcraft December 2023 Web Server Survey stats.
author Yaroslav Zhuravlev <yar@nginx.com>
date Wed, 20 Dec 2023 10:42:01 +0000
parents ac131944d349
children
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1 <!--
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2 Copyright (C) Igor Sysoev
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3 Copyright (C) Nginx, Inc.
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4 -->
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5
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6 <!DOCTYPE article SYSTEM "../../../dtd/article.dtd">
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7
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8 <article name="Setting up hashes"
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9 link="/en/docs/hash.html"
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10 lang="en"
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11 rev="1">
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13 <section>
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14
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15 <para>
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16 To quickly process static sets of data such as server names,
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17 <link doc="http/ngx_http_map_module.xml" id="map"/> directive’s values,
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18 MIME types, names of request header strings,
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19 nginx uses hash tables.
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20 During the start and each re-configuration nginx selects the
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21 minimum possible sizes of hash tables such that the bucket size
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22 that stores keys with identical hash values does not exceed the
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23 configured parameter (hash bucket size).
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24 The size of a table is expressed in buckets.
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25 The adjustment is continued until the table size exceeds the
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26 hash max size parameter.
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27 Most hashes have the corresponding directives that allow changing
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28 these parameters, for example, for the server names hash they are
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29 <link doc="http/ngx_http_core_module.xml" id="server_names_hash_max_size"/>
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30 and <link doc="http/ngx_http_core_module.xml" id="server_names_hash_bucket_size"/>.
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31 </para>
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32
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33 <para>
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34 The hash bucket size parameter is aligned to the size that is a
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35 multiple of the processor’s cache line size. This speeds up
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36 key search in a hash on modern processors by reducing the number
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37 of memory accesses.
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38 If hash bucket size is equal to one processor’s cache line size
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39 then the number of memory accesses during the key search will be
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40 two in the worst case&mdash;first to compute the bucket address,
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41 and second during the key search inside the bucket.
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42 Therefore, if nginx emits the message requesting to increase
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43 either hash max size or hash bucket size then the first parameter
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44 should first be increased.
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45 </para>
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47 </section>
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48
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49 </article>