Table of Contents
The Memory Pool subsystem is the general-purpose shared memory buffer pool used by Berkeley DB. This module is useful outside of the Berkeley DB package for processes that require page-oriented, shared and cached file access. (However, such "use outside of Berkeley DB" is not supported in replicated environments.)
A memory pool is a memory cache shared among any number of threads of control. The DB_INIT_MPOOL flag to the DB_ENV->open() method opens and optionally creates a memory pool. When that pool is no longer in use, it should be closed using the DB_ENV->close() method.
The DB_ENV->memp_fcreate() method returns a DB_MPOOLFILE handle on an underlying file within the memory pool. The file may be opened using the DB_MPOOLFILE->open() method. The DB_MPOOLFILE->get() method is used to retrieve pages from files in the pool. All retrieved pages must be subsequently returned using the DB_MPOOLFILE->put() method. At the time pages are returned, they may be marked 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 DB_MPOOLFILE->sync() method. When the file handle is no longer in use, it should be closed using the DB_MPOOLFILE->close() method.
There are additional configuration interfaces that apply when opening a new file in the memory pool:
There are additional interfaces for the memory pool as a whole:
| Memory Pools and Related Methods | Description |
|---|---|
| DB->get_mpf() handle | Return DB's underlying DB_MPOOLFILE handle |
| DB_ENV->memp_stat() | Return memory pool statistics |
| DB_ENV->memp_sync() | Flush pages from a memory pool |
| DB_ENV->memp_trickle() | Trickle flush pages from a memory pool |
| Memory Pool Configuration | |
| DB_ENV->memp_register() | Register input/output functions for a file in a memory pool |
| DB_ENV->set_cachesize() | Set the environment cache size |
| DB_ENV->set_cache_max() | Set the maximum cache size |
| DB_ENV->set_mp_max_openfd() | Set the maximum number of open file descriptors |
| DB_ENV->set_mp_max_write() | Set the maximum number of sequential disk writes |
| DB_ENV->set_mp_mmapsize() | Set maximum mapped-in database file size |
| Memory Pool Files | |
| DB_ENV->memp_fcreate() | Create a memory pool file handle |
| DB_MPOOLFILE->close() | Close a file in a memory pool |
| DB_MPOOLFILE->get() | Get page from a file in a memory pool |
| DB_MPOOLFILE->open() | Open a file in a memory pool |
| DB_MPOOLFILE->put() | Return a page to a memory pool |
| DB_MPOOLFILE->sync() | Flush pages from a file in a memory pool |
| Memory Pool File Configuration | |
| DB_MPOOLFILE->set_clear_len() | Set file page bytes to be cleared |
| DB_MPOOLFILE->set_fileid() | Set file unique identifier |
| DB_MPOOLFILE->set_flags() | General memory pool file configuration |
| DB_MPOOLFILE->set_ftype() | Set file type |
| DB_MPOOLFILE->set_lsn_offset() | Set file log-sequence-number offset |
| DB_MPOOLFILE->set_maxsize() | Set maximum file size |
| DB_MPOOLFILE->set_pgcookie() | Set file cookie for pgin/pgout |
| mempset_priority() | Set memory pool file priority |