intro.so

berkeley db 4.6.21的源码。berkeley db是一个简单的数据库管理系统

m4_comment([$Id: intro.so,v 10.26 2007/04/05 20:54:56 bostic Exp $])

m4_ref_title(Memory Pool Subsystem,
    Introduction to the memory pool subsystem,
    [introduction to the @memory pool subsystem,
    introduction to the memory @cache subsystem,
    introduction to the @buffer pool subsystem], log/limits, mp/config)

m4_p([dnl
The Memory Pool subsystem is the general-purpose shared memory buffer
pool used by m4_db.  This module is useful outside of the m4_db package
for processes that require page-oriented, shared and cached file access.])

m4_p([dnl
A m4_italic(memory pool) is a memory cache shared among any number of
threads of control.  The m4_ref(DB_INIT_MPOOL) flag to the
m4_refT(dbenv_open) opens and optionally creates a memory pool.  When
that pool is no longer in use, it should be closed using the
m4_refT(dbenv_close).])

m4_p([dnl
The m4_refT(memp_fcreate) returns a m4_ref(DbMpoolFile) handle on an
underlying file within the memory pool.  The file may be opened using
the m4_refT(memp_fopen).  The m4_refT(memp_fget) is used to retrieve
pages from files in the pool.  All retrieved pages must be subsequently
returned using the m4_refT(memp_fput).  At the time pages are returned,
they may be marked m4_bold(dirty), which causes them to be written to
the underlying file before being discarded from the pool.  If there is
insufficient room to bring a new page in the pool, a page is selected
to be discarded from the pool using a least-recently-used algorithm.
All dirty pages in the pool from the file may be flushed using the
m4_refT(memp_fsync).  When the file handle is no longer in use, it
should be closed using the m4_refT(memp_fclose).])

m4_p([dnl
There are additional configuration interfaces that apply when opening
a new file in the memory pool:])

m4_bulletbegin
m4_bullet([dnl
The m4_refT(memp_set_clear_len) specifies the number of bytes to clear
when creating a new page in the memory pool.])

m4_bullet([dnl
The m4_refT(memp_set_fileid) specifies a unique ID associated with the file.])

m4_bullet([dnl
The m4_refT(memp_set_ftype) specifies the type of file for the purposes of
page input and output processing.])

m4_bullet([dnl
The m4_refT(memp_set_lsn_offset) specifies the byte offset of each page's
log sequence number (m4_ref(DbLsn)) for the purposes of transaction
checkpoints.])

m4_bullet([dnl
The m4_refT(memp_set_pgcookie) specifies an application provided argument
for the purposes of page input and output processing.])

m4_bulletend

m4_p([dnl
There are additional interfaces for the memory pool as a whole:])

m4_bulletbegin
m4_bullet([dnl
It is possible to gradually flush buffers from the pool in order to
maintain a consistent percentage of clean buffers in the pool using
the m4_refT(memp_trickle).])

m4_bullet([dnl
Because special-purpose processing may be necessary when pages are read
or written (for example, endian conversion, or page checksums), the
m4_ref(memp_register) function allows applications to specify automatic
input and output processing in these cases.])

m4_bullet([dnl
The m4_ref(db_stat) utility uses the m4_refT(memp_stat) to display
statistics about the efficiency of the pool.])

m4_bullet([dnl
All dirty pages in the pool may be flushed using the m4_refT(memp_sync).
In addition, m4_ref(memp_sync) takes an argument that is specific to
database systems, and which allows the memory pool to be flushed up to
a specified log sequence number (m4_ref(DbLsn)).])

m4_bullet([dnl
The entire pool may be discarded using the m4_refT(dbenv_remove).])
m4_bulletend

include(memp/m4.methods)

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