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Yasuhiro KIMURA
120 lines
6.4 KiB
HTML
120 lines
6.4 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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<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 19. The Memory Pool Subsystem" />
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<link rel="prev" href="mp.html" title="Chapter 19. 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 18.1.40</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 19. The Memory Pool Subsystem </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>
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There are two issues to consider when configuring the memory
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pool.
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</p>
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<p>
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The first issue, the most important tuning parameter for
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Berkeley DB applications, is the size of the memory pool.
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There are two ways to specify the pool size. First, calling
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the <a href="../api_reference/C/envset_cachesize.html" class="olink">DB_ENV->set_cachesize()</a> method specifies the pool size for all
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of the applications sharing the Berkeley DB environment.
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Second, the <a href="../api_reference/C/dbset_cachesize.html" class="olink">DB->set_cachesize()</a> method only specifies a pool
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size for the specific database. Note: It is meaningless to
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call <a href="../api_reference/C/dbset_cachesize.html" class="olink">DB->set_cachesize()</a> for a database opened inside of a
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Berkeley DB environment because the environment pool size will
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override any pool size specified for a single database. For
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information on tuning the Berkeley DB cache size, see <a class="xref" href="general_am_conf.html#am_conf_cachesize" title="Selecting a cache size">Selecting a cache size</a>.
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</p>
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<p>
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Note the memory pool defaults to assuming that the average
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page size is 4k. This factor is used to determine the size of
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the hash table used to locate pages in the memory pool. The
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size of the hash table is calculated to so that on average 2.5
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pages will be in each hash table entry. Each page requires a
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mutex be allocated to it and the average page size is used to
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determine the number of mutexes to allocate to the memory
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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
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values for the page size, but in some cases you may be able to
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achieve better performance by manually configuring the page
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size. The expected page size, hash table size and mutex count
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can be set via the methods: <a href="../api_reference/C/envset_mp_pagesize.html" class="olink">DB_ENV->set_mp_pagesize()</a>,
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<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>
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The second memory pool configuration issue is the maximum
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size an underlying file can be and still be mapped into the
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process address space (instead of reading the file's pages
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into the cache). Mapping files into the process address space
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can result in better performance because available virtual
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memory is often much larger than the local cache, and page
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faults are faster than page copying on many systems. However,
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in the presence of limited virtual memory, it can cause
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resource starvation; and in the presence of large databases,
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it can result in immense process sizes. In addition, because
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of the requirements of the Berkeley DB transactional
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implementation, only read-only files can be mapped into
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process memory.
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</p>
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<p>
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To specify that no files are to be mapped into the process
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address 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
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should not be mapped into the process address space, specify
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the <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
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the <a href="../api_reference/C/envset_mp_mmapsize.html" class="olink">DB_ENV->set_mp_mmapsize()</a> method.
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</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 19. The Memory Pool Subsystem </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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