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110 lines
6.1 KiB
HTML
<?xml version="1.0" encoding="UTF-8" standalone="no"?>
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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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<html xmlns="http://www.w3.org/1999/xhtml">
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
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<title>Configuring the memory pool</title>
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<link rel="stylesheet" href="gettingStarted.css" type="text/css" />
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<meta name="generator" content="DocBook XSL Stylesheets V1.73.2" />
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<link rel="start" href="index.html" title="Berkeley DB Programmer's Reference Guide" />
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<link rel="up" href="mp.html" title="Chapter 18. The Memory Pool Subsystem" />
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<link rel="prev" href="mp.html" title="Chapter 18. The Memory Pool Subsystem" />
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<body>
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<div xmlns="" class="navheader">
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<div class="libver">
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<p>Library Version 11.2.5.3</p>
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</div>
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<table width="100%" summary="Navigation header">
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<tr>
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<th colspan="3" align="center">Configuring the memory pool</th>
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</tr>
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<tr>
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<td width="20%" align="left"><a accesskey="p" href="mp.html">Prev</a> </td>
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<th width="60%" align="center">Chapter 18.
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The Memory Pool Subsystem
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</th>
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<td width="20%" align="right"> <a accesskey="n" href="mp_warm.html">Next</a></td>
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</tr>
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</table>
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<hr />
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</div>
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<div class="sect1" lang="en" xml:lang="en">
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<div class="titlepage">
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<div>
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<div>
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<h2 class="title" style="clear: both"><a id="mp_config"></a>Configuring the memory pool</h2>
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</div>
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</div>
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</div>
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<p>There are two issues to consider when configuring the memory pool.</p>
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<p>The first issue, the most important tuning parameter for Berkeley DB
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applications, is the size of the memory pool. There are two ways to
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specify the pool size. First, calling the <a href="../api_reference/C/envset_cachesize.html" class="olink">DB_ENV->set_cachesize()</a> method
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specifies the pool size for all of the applications sharing the Berkeley DB
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environment. Second, the <a href="../api_reference/C/dbset_cachesize.html" class="olink">DB->set_cachesize()</a> method only specifies a
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pool size for the specific database. Note: It is meaningless to call
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<a href="../api_reference/C/dbset_cachesize.html" class="olink">DB->set_cachesize()</a> for a database opened inside of a Berkeley DB
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environment because the environment pool size will override any pool
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size specified for a single database. For information on tuning the
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Berkeley DB cache size, see
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<a class="xref" href="general_am_conf.html#am_conf_cachesize" title="Selecting a cache size">Selecting a cache size</a>.</p>
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<p>
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Note the memory pool defaults to assuming that the average page size is
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4k. This factor is used to determine the size of the hash table used
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to locate pages in the memory pool. The size of the hash table is
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calculated to so that on average 2.5 pages will be in each hash table
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entry. Each page requires a mutex be allocated to it and the average
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page size is used to determine the number of mutexes to allocate to the
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memory pool.
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</p>
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<p>
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Normally you should see good results by using the default values for
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the page size, but in some cases you may be able to achieve better
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performance by manually configuring the page size. The expected page
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size, hash table size and mutex count can be set via the methods:
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<a href="../api_reference/C/envset_mp_pagesize.html" class="olink">DB_ENV->set_mp_pagesize()</a>, <a href="../api_reference/C/envset_mp_tablesize.html" class="olink">DB_ENV->set_mp_tablesize()</a>, and <a href="../api_reference/C/envset_mp_mtxcount.html" class="olink">DB_ENV->set_mp_mtxcount()</a>.
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</p>
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<p>The second memory pool configuration issue is the maximum size an
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underlying file can be and still be mapped into the process address
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space (instead of reading the file's pages into the cache). Mapping
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files into the process address space can result in better performance
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because available virtual memory is often much larger than the local
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cache, and page faults are faster than page copying on many systems.
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However, in the presence of limited virtual memory, it can cause
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resource starvation; and in the presence of large databases, it can
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result in immense process sizes. In addition, because of the
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requirements of the Berkeley DB transactional implementation, only read-only
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files can be mapped into process memory.</p>
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<p>To specify that no files are to be mapped into the process address
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space, specify the <a href="../api_reference/C/dbopen.html#open_DB_NOMMAP" class="olink">DB_NOMMAP</a> flag to the
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<a href="../api_reference/C/envset_flags.html" class="olink">DB_ENV->set_flags()</a> method. To specify that any individual file should
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not be mapped into the process address space, specify the
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<a href="../api_reference/C/dbopen.html#open_DB_NOMMAP" class="olink">DB_NOMMAP</a> flag to the <a href="../api_reference/C/mempfopen.html" class="olink">DB_MPOOLFILE->open()</a> interface. To limit
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the size of files mapped into the process address space, use the
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<a href="../api_reference/C/envset_mp_mmapsize.html" class="olink">DB_ENV->set_mp_mmapsize()</a> method.</p>
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</div>
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<div class="navfooter">
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<hr />
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<table width="100%" summary="Navigation footer">
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<tr>
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<td width="40%" align="left"><a accesskey="p" href="mp.html">Prev</a> </td>
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<td width="20%" align="center">
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<a accesskey="u" href="mp.html">Up</a>
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</td>
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<td width="40%" align="right"> <a accesskey="n" href="mp_warm.html">Next</a></td>
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</tr>
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<tr>
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<td width="40%" align="left" valign="top">Chapter 18.
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The Memory Pool Subsystem
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</td>
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<td width="20%" align="center">
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<a accesskey="h" href="index.html">Home</a>
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</td>
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<td width="40%" align="right" valign="top"> Warming the memory pool</td>
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</tr>
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</table>
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</div>
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</body>
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</html>
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