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<h2 class="title" style="clear: both"><a id="env_size"></a>Sizing a database environment</h2>
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<p>
The Berkeley DB environment allocates memory to hold shared
structures, either in shared regions or in process data space (if the
<a href="../api_reference/C/envopen.html#envopen_DB_PRIVATE" class="olink">DB_PRIVATE</a> flag is specified). There are three distinct memory
regions:
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The memory pool (also known as the database page cache),
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the area containing mutexes, and
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the main region which holds all other shared structures.
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<p>
The shared structures in the main region are used by the lock,
transaction, logging, thread and replicatoin subsystems.
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<p>
Determining the amount of space allocated for each of these shared
structures is dependent upon the structure in question. The sizing
of the memory pool is discussed in
<a class="xref" href="mp_config.html" title="Configuring the memory pool">Configuring the memory pool</a>. The amount
of memory needed for mutexes is calculated from the number of
mutexes needed by various subsystems and can be adjusted using the
<a href="../api_reference/C/mutexset_increment.html" class="olink">DB_ENV->mutex_set_increment()</a> method.
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<p>
For applications using shared memory (that is, they do not specify
<a href="../api_reference/C/envopen.html#envopen_DB_PRIVATE" class="olink">DB_PRIVATE</a>), a maximum memory size for the main region must be
specified or left to default. The maximum memory size is specified
using the <a href="../api_reference/C/envset_memory_max.html" class="olink">DB_ENV->set_memory_max()</a> method.
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The amount of memory needed by an application is dependent
on the resources that the application uses. For a very rough
estimate, add all of the following together:
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The environment has an overhead of about 80 kilobytes
without statistics enabled or 250 kilobytes with statistics
enabled.
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<p>
Identify the amount of space you require for your locks:
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Estimate the number of threads of control that will
simultaneously access the environment.
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<p>
Estimate the number of concurrency locks that, on average,
will be required by each thread. For information on
sizing concurrency locks, see
<a class="xref" href="lock_max.html" title="Configuring locking: sizing the system">Configuring locking: sizing the system</a>.
</p>
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<li>
<p>
Multiply these two numbers, then multiply by 1/2 to
arrive at the number of kilobytes required to
service your locks.
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</ol>
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</li>
<li>
<p>
Estimate the number of open database handles you will use
at any given time. For each database handle, there is an
overhead of about 1/2 kilobyte.
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</li>
<li>
<p>
Add 1 kilobyte for each active transaction.
</p>
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<p>
Note that these are very rough guidelines. It is best to
overestimate the needs of your applications, because if the memory
allocation is exhausted the application must be shutdown to
increase the allocation.
</p>
<p>
The estimate for maximum memory need not be exact. In most
situations there is little penalty for over estimating. For systems
using memory mapped files for the shared environment, this only
allocates the address space in the process to hold the maximum
memory. The backing file will only be extended as needed. For
systems running with <a href="../api_reference/C/envopen.html#envopen_DB_PRIVATE" class="olink">DB_PRIVATE</a> specified, the maximum memory
serves only as a limit and memory is allocated from the process
data space as needed. No maximum need be set for private
environments.
</p>
<p>
For locking and thread information, groups of objects are allocated
when needed so that there is less contention in the allocator
during performance critical operations. Once allocated to a
particular use, this memory will only be used for that structure.
To avoid runtime contention, or to ensure a minimum number of a
particular type of object, the <a href="../api_reference/C/envset_memory_init.html" class="olink">DB_ENV->set_memory_init()</a> method can be
used. This method can set the initial numbers of particular types
of structures to allocate at environment creation time.
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