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6<title>Berkeley DB Reference Guide: Introduction to the memory pool subsystem</title>
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13<td><b><dl><dt>Berkeley DB Reference Guide:<dd>Memory Pool Subsystem</dl></b></td>
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16<p align=center><b>Introduction to the memory pool subsystem</b></p>
17<p>The Memory Pool subsystem is the general-purpose shared memory buffer
18pool used by Berkeley DB.  This module is useful outside of the Berkeley DB package
19for processes that require page-oriented, shared and cached file access.</p>
20<p>A <i>memory pool</i> is a memory cache shared among any number of
21threads of control.  The <a href="../../api_c/env_open.html#DB_INIT_MPOOL">DB_INIT_MPOOL</a> flag to the
22<a href="../../api_c/env_open.html">DB_ENV-&gt;open</a> method opens and optionally creates a memory pool.  When
23that pool is no longer in use, it should be closed using the
24<a href="../../api_c/env_close.html">DB_ENV-&gt;close</a> method.</p>
25<p>The <a href="../../api_c/memp_fcreate.html">DB_ENV-&gt;memp_fcreate</a> method returns a <a href="../../api_c/mempfile_class.html">DB_MPOOLFILE</a> handle on an
26underlying file within the memory pool.  The file may be opened using
27the <a href="../../api_c/memp_fopen.html">DB_MPOOLFILE-&gt;open</a> method.  The <a href="../../api_c/memp_fget.html">DB_MPOOLFILE-&gt;get</a> method is used to retrieve
28pages from files in the pool.  All retrieved pages must be subsequently
29returned using the <a href="../../api_c/memp_fput.html">DB_MPOOLFILE-&gt;put</a> method.  At the time pages are returned,
30they may be marked <b>dirty</b>, which causes them to be written to
31the underlying file before being discarded from the pool.  If there is
32insufficient room to bring a new page in the pool, a page is selected
33to be discarded from the pool using a least-recently-used algorithm.
34All dirty pages in the pool from the file may be flushed using the
35<a href="../../api_c/memp_fsync.html">DB_MPOOLFILE-&gt;sync</a> method.  When the file handle is no longer in use, it
36should be closed using the <a href="../../api_c/memp_fclose.html">DB_MPOOLFILE-&gt;close</a> method.</p>
37<p>There are additional configuration interfaces that apply when opening
38a new file in the memory pool:</p>
39<p><ul type=disc>
40<li>The <a href="../../api_c/memp_set_clear_len.html">DB_MPOOLFILE-&gt;set_clear_len</a> method specifies the number of bytes to clear
41when creating a new page in the memory pool.
42<li>The <a href="../../api_c/memp_set_fileid.html">DB_MPOOLFILE-&gt;set_fileid</a> method specifies a unique ID associated with the file.
43<li>The <a href="../../api_c/memp_set_ftype.html">DB_MPOOLFILE-&gt;set_ftype</a> method specifies the type of file for the purposes of
44page input and output processing.
45<li>The <a href="../../api_c/memp_set_lsn_offset.html">DB_MPOOLFILE-&gt;set_lsn_offset</a> method specifies the byte offset of each page's
46log sequence number (<a href="../../api_c/lsn_class.html">DB_LSN</a>) for the purposes of transaction
47checkpoints.
48<li>The <a href="../../api_c/memp_set_pgcookie.html">DB_MPOOLFILE-&gt;set_pgcookie</a> method specifies an application provided argument
49for the purposes of page input and output processing.
50</ul>
51<p>There are additional interfaces for the memory pool as a whole:</p>
52<p><ul type=disc>
53<li>It is possible to gradually flush buffers from the pool in order to
54maintain a consistent percentage of clean buffers in the pool using
55the <a href="../../api_c/memp_trickle.html">DB_ENV-&gt;memp_trickle</a> method.
56<li>Because special-purpose processing may be necessary when pages are read
57or written (for example, endian conversion, or page checksums), the
58<a href="../../api_c/memp_register.html">DB_ENV-&gt;memp_register</a> function allows applications to specify automatic
59input and output processing in these cases.
60<li>The <a href="../../utility/db_stat.html">db_stat</a> utility uses the <a href="../../api_c/memp_stat.html">DB_ENV-&gt;memp_stat</a> method to display
61statistics about the efficiency of the pool.
62<li>All dirty pages in the pool may be flushed using the <a href="../../api_c/memp_sync.html">DB_ENV-&gt;memp_sync</a> method.
63In addition, <a href="../../api_c/memp_sync.html">DB_ENV-&gt;memp_sync</a> takes an argument that is specific to
64database systems, and which allows the memory pool to be flushed up to
65a specified log sequence number (<a href="../../api_c/lsn_class.html">DB_LSN</a>).
66<li>The entire pool may be discarded using the <a href="../../api_c/env_remove.html">DB_ENV-&gt;remove</a> method.
67</ul>
68<!--$Id: m4.methods,v 1.11 2007/07/06 00:22:53 mjc Exp $-->
69<table border=1 align=center>
70<tr><th>Memory Pools and Related Methods</th><th>Description</th></tr>
71<!--Db::mpf--><tr><td><a href="../../api_c/db_get_mpf.html">DB-&gt;get_mpf</a></td><td>Return <a href="../../api_c/db_class.html">DB</a>'s underlying <a href="../../api_c/mempfile_class.html">DB_MPOOLFILE</a> handle</td></tr>
72<!--DbEnv::memp_stat--><tr><td><a href="../../api_c/memp_stat.html">DB_ENV-&gt;memp_stat</a></td><td>Return memory pool statistics</td></tr>
73<!--DbEnv::memp_sync--><tr><td><a href="../../api_c/memp_sync.html">DB_ENV-&gt;memp_sync</a></td><td>Flush pages from a memory pool</td></tr>
74<!--DbEnv::memp_trickle--><tr><td><a href="../../api_c/memp_trickle.html">DB_ENV-&gt;memp_trickle</a></td><td>Trickle flush pages from a memory pool</td></tr>
75<!--DbMemoryException-->
76<tr><th>Memory Pool Configuration</th><th><br></th></tr>
77<!--DbEnv::memp_register--><tr><td><a href="../../api_c/memp_register.html">DB_ENV-&gt;memp_register</a></td><td>Register input/output functions for a file in a memory pool</td></tr>
78<!--DbEnv::set_cachesize--><tr><td><a href="../../api_c/env_set_cachesize.html">DB_ENV-&gt;set_cachesize</a></td><td>Set the environment cache size</td></tr>
79<!--DbEnv::set_cache_max--><tr><td><a href="../../api_c/env_set_cache_max.html">DB_ENV-&gt;set_cache_max</a></td><td>Set the maximum cache size</td></tr>
80<!--DbEnv::set_mp_max_openfd--><tr><td><a href="../../api_c/memp_openfd.html">DB_ENV-&gt;set_mp_max_openfd</a></td><td>Set the maximum number of open file descriptors</td></tr>
81<!--DbEnv::set_mp_max_write--><tr><td><a href="../../api_c/memp_maxwrite.html">DB_ENV-&gt;set_mp_max_write</a></td><td>Set the maximum number of sequential disk writes</td></tr>
82<!--DbEnv::set_mp_mmapsize--><tr><td><a href="../../api_c/env_set_mp_mmapsize.html">DB_ENV-&gt;set_mp_mmapsize</a></td><td>Set maximum mapped-in database file size</td></tr>
83<tr><th>Memory Pool Files</th><th><br></th></tr>
84<!--DbEnv::memp_fcreate--><tr><td><a href="../../api_c/memp_fcreate.html">DB_ENV-&gt;memp_fcreate</a></td><td>Create a memory pool file handle</td></tr>
85<!--DbMpoolFile::-->
86<!--DbMpoolFile::close--><tr><td><a href="../../api_c/memp_fclose.html">DB_MPOOLFILE-&gt;close</a></td><td>Close a file in a memory pool</td></tr>
87<!--DbMpoolFile::get--><tr><td><a href="../../api_c/memp_fget.html">DB_MPOOLFILE-&gt;get</a></td><td>Get page from a file in a memory pool</td></tr>
88<!--DbMpoolFile::open--><tr><td><a href="../../api_c/memp_fopen.html">DB_MPOOLFILE-&gt;open</a></td><td>Open a file in a memory pool</td></tr>
89<!--DbMpoolFile::put--><tr><td><a href="../../api_c/memp_fput.html">DB_MPOOLFILE-&gt;put</a></td><td>Return a page to a memory pool</td></tr>
90<!--DbMpoolFile::sync--><tr><td><a href="../../api_c/memp_fsync.html">DB_MPOOLFILE-&gt;sync</a></td><td>Flush pages from a file in a memory pool</td></tr>
91<tr><th>Memory Pool File Configuration</th><th><br></th></tr>
92<!--DbMpoolFile::set_clear_len--><tr><td><a href="../../api_c/memp_set_clear_len.html">DB_MPOOLFILE-&gt;set_clear_len</a></td><td>Set file page bytes to be cleared</td></tr>
93<!--DbMpoolFile::set_fileid--><tr><td><a href="../../api_c/memp_set_fileid.html">DB_MPOOLFILE-&gt;set_fileid</a></td><td>Set file unique identifier</td></tr>
94<!--DbMpoolFile::set_flags--><tr><td><a href="../../api_c/memp_set_flags.html">DB_MPOOLFILE-&gt;set_flags</a></td><td>General memory pool file configuration</td></tr>
95<!--DbMpoolFile::set_ftype--><tr><td><a href="../../api_c/memp_set_ftype.html">DB_MPOOLFILE-&gt;set_ftype</a></td><td>Set file type</td></tr>
96<!--DbMpoolFile::set_lsn_offset--><tr><td><a href="../../api_c/memp_set_lsn_offset.html">DB_MPOOLFILE-&gt;set_lsn_offset</a></td><td>Set file log-sequence-number offset</td></tr>
97<!--DbMpoolFile::set_maxsize--><tr><td><a href="../../api_c/memp_set_maxsize.html">DB_MPOOLFILE-&gt;set_maxsize</a></td><td>Set maximum file size</td></tr>
98<!--DbMpoolFile::set_pgcookie--><tr><td><a href="../../api_c/memp_set_pgcookie.html">DB_MPOOLFILE-&gt;set_pgcookie</a></td><td>Set file cookie for pgin/pgout</td></tr>
99<!--DbMpoolFile::set_priority--><tr><td><a href="../../api_c/memp_set_priority.html">DB_MPOOLFILE-&gt;set_priority</a></td><td>Set memory pool file priority</td></tr>
100</table>
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