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Berkeley DB Reference Guide:
  Memory Pool Subsystem

# Configuring the memory pool

There are two issues to consider when configuring the memory pool.

The first issue, the most important tuning parameter for Berkeley DB applications, is the size of the memory pool. There are two ways to specify the pool size. First, calling the DB_ENV->set_cachesize function specifies the pool size for all of the applications sharing the Berkeley DB environment. Second, the DB->set_cachesize function only specifies a pool size for the specific database. Note: It is meaningless to call DB->set_cachesize for a database opened inside of a Berkeley DB environment because the environment pool size will override any pool size specified for a single database. For information on tuning the Berkeley DB cache size, see Selecting a cache size.

The second memory pool configuration issue is the maximum size an underlying file can be and still be mapped into the process address space (instead of reading the file's pages into the cache). Mapping files into the process address space can result in better performance because available virtual memory is often much larger than the local cache, and page faults are faster than page copying on many systems. However, in the presence of limited virtual memory, it can cause resource starvation; and in the presence of large databases, it can result in immense process sizes. In addition, because of the requirements of the Berkeley DB transactional implementation, only read-only files can be mapped into process memory.

To specify that no files are to be mapped into the process address space, specify the DB_NOMMAP flag to the DB_ENV->set_flags interface. To specify that any individual file should not be mapped into the process address space, specify the DB_NOMMAP flag to the memp_fopen interface. To limit the size of files mapped into the process address space, use the DB_ENV->set_mp_mmapsize function.

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