m4_comment([$Id: config.so,v 10.20 2003/10/18 19:16:04 bostic Exp $])

m4_ref_title(Memory Pool Subsystem,
    Configuring the memory pool,
    @memory pool configuration, mp/intro, txn/intro)

m4_p([dnl
There are two issues to consider when configuring the memory pool.])

m4_p([dnl
The first issue, the most important tuning parameter for m4_db
applications, is the size of the memory pool.  There are two ways to
specify the pool size.  First, calling the m4_refT(dbenv_set_cachesize)
specifies the pool size for all of the applications sharing the m4_db
environment.  Second, the m4_refT(dbh_set_cachesize) only specifies a
pool size for the specific database.  Note: It is meaningless to call
m4_ref(dbh_set_cachesize) for a database opened inside of a m4_db
environment because the environment pool size will override any pool
size specified for a single database.  For information on tuning the
m4_db cache size, see m4_link(M4RELDIR/ref/am_conf/cachesize, [Selecting
a cache size]).])

m4_p([dnl
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 m4_db transactional implementation, only read-only
files can be mapped into process memory.])

m4_p([dnl
To specify that no files are to be mapped into the process address
space, specify the m4_ref(DB_NOMMAP) flag to the
m4_refT(dbenv_set_flags).  To specify that any individual file should
not be mapped into the process address space, specify the
m4_ref(DB_NOMMAP) flag to the m4_ref(memp_fopen) interface.  To limit
the size of files mapped into the process address space, use the
m4_refT(dbenv_set_mp_mmapsize).])

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