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Added copyright markers to documentation sources.
author Ruslan Ermilov <ru@nginx.com>
date Tue, 10 Jul 2012 12:59:42 +0000
parents a1071d0d0979
children 764fbac1b8b4
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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
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12 <section>
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13
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14 <para>
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15 To quickly process static sets of data such as server names,
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16 <link doc="http/ngx_http_map_module.xml" id="map"/> directive's values,
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17 MIME types, names of request header strings,
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18 nginx uses hash tables.
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19 During the start and each re-configuration nginx selects the
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20 minimum possible sizes of hash tables such that the bucket size
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21 that stores keys with identical hash values does not exceed the
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22 configured parameter (hash bucket size).
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23 The size of a table is expressed in buckets.
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24 The adjustment is continued until the table size exceeds the
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25 hash max size parameter.
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26 Most hashes have the corresponding directives that allow to change
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27 these parameters, for example, for the server names hash they are
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28 <link doc="http/ngx_http_core_module.xml" id="server_names_hash_max_size"/>
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29 and <link doc="http/ngx_http_core_module.xml" id="server_names_hash_bucket_size"/>.
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30 </para>
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31
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32 <para>
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33 The hash bucket size parameter is aligned to the size that is a
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34 multiple of the processor's cache line size. This speeds up
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35 key search in a hash on modern processors by reducing the number
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36 of memory accesses.
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37 If hash bucket size is equal to one processor's cache line size
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38 then the number of memory accesses during the key search will be
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39 two in the worst case&mdash;first to compute the bucket address,
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40 and second during the key search inside the bucket.
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41 Therefore, if nginx emits the message requesting to increase
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42 either hash max size or hash bucket size then the first parameter
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43 should first be increased.
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44 </para>
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45
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46 </section>
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47
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48 </article>