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<title>Berkeley DB Reference Guide: Configuring locking: sizing the system</title>
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<td><h3><dl><dt>Berkeley DB Reference Guide:<dd>Locking Subsystem</dl></h3></td>
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<h1 align=center>Configuring locking: sizing the system</h1>
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<p>The lock system is sized using the following three functions:
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<p><blockquote><pre><a href="../../api_c/env_set_lk_max_locks.html">DB_ENV->set_lk_max_locks</a>
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<a href="../../api_c/env_set_lk_max_lockers.html">DB_ENV->set_lk_max_lockers</a>
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<a href="../../api_c/env_set_lk_max_objects.html">DB_ENV->set_lk_max_objects</a></pre></blockquote>
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<p>The <a href="../../api_c/env_set_lk_max_locks.html">DB_ENV->set_lk_max_locks</a>, <a href="../../api_c/env_set_lk_max_lockers.html">DB_ENV->set_lk_max_lockers</a>,
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and <a href="../../api_c/env_set_lk_max_objects.html">DB_ENV->set_lk_max_objects</a> methods specify the maximum number of
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locks, lockers, and locked objects supported by the lock subsystem,
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respectively. The maximum number of locks is the number of locks that
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can be simultaneously requested in the system. The maximum number of
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lockers is the number of lockers that can simultaneously request locks
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in the system. The maximum number of lock objects is the number of
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objects that can simultaneously be locked in the system. Selecting
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appropriate values requires an understanding of your application and
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its databases. If the values are too small, requests for locks in an
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application will fail. If the values are too large, the locking
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subsystem will consume more resources than is necessary. It is better
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to err in the direction of allocating too many locks, lockers, and
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objects because increasing the number of locks does not require large
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amounts of additional resources.
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<p>The recommended algorithm for selecting the maximum number of locks,
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lockers, and lock objects is to run the application under stressful
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conditions and then review the lock system's statistics to determine
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the maximum number of locks, lockers, and lock objects that were used.
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Then, double these values for safety. However, in some large
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applications, finer granularity of control is necessary in order to
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minimize the size of the Lock subsystem.
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<p>The maximum number of lockers can be estimated as follows:
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<li>If the database environment is configured to use transactions, the
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maximum number of lockers needed is the number of simultaneously active
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transactions and child transactions (where a child transaction is active
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until its parent commits or aborts, not until it commits or aborts).
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<li>If the database environment is not configured to use transactions, the
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maximum number of lockers needed is the number of simultaneous
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non-cursor operations plus an additional locker for every simultaneously
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<p>The maximum number of lock objects needed can be estimated as follows:
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<li>For Btree and Recno access methods, you will need one lock object per
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level of the database tree, at a minimum. (Unless keys are quite large
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with respect to the page size, neither Recno nor Btree database trees
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should ever be deeper than five levels.) Then, you will need one lock
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object for each leaf page of the database tree that will be
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simultaneously accessed.
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<li>For the Queue access method, you will need one lock object per record
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that is simultaneously accessed. To this, add one lock object per page
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that will be simultaneously accessed. (Because the Queue access method
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uses fixed-length records and the database page size is known, it is
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possible to calculate the number of pages -- and, therefore, the lock
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objects -- required.) Deleted records skipped by a <a href="../../api_c/dbc_get.html#DB_NEXT">DB_NEXT</a> or
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<a href="../../api_c/dbc_get.html#DB_PREV">DB_PREV</a> operation do not require a separate lock object.
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Further, if your application is using transactions, no database
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operation will ever use more than three lock objects at any time.
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<li>For the Hash access method, you only need a single lock object.
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<p>For all access methods, you should then add an additional lock object
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per database for the database's metadata page.
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<p>The maximum number of locks required by an application cannot be easily
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estimated. It is possible to calculate a maximum number of locks by
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multiplying the maximum number of lockers, times the maximum number of
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lock objects, times two (two for the two possible lock modes for each
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object, read and write). However, this is a pessimal value, and real
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applications are unlikely to actually need that many locks. Reviewing
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the Lock subsystem statistics is the best way to determine this value.
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