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<h2 class="title" style="clear: both"><a id="lock_max"></a>Configuring locking: sizing the system</h2>
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<p>The lock system is sized using the following four methods:</p>
<div class="orderedlist">
<ol type="1">
<li>
<p>
<a href="../api_reference/C/envset_lk_max_locks.html" class="olink">DB_ENV->set_lk_max_locks()</a>
</p>
</li>
<li>
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<a href="../api_reference/C/envset_lk_max_lockers.html" class="olink">DB_ENV->set_lk_max_lockers()</a>
</p>
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<a href="../api_reference/C/envset_lk_max_objects.html" class="olink">DB_ENV->set_lk_max_objects()</a>
</p>
</li>
<li>
<p>
<a href="../api_reference/C/envset_lk_partitions.html" class="olink">DB_ENV->set_lk_partitions()</a>
</p>
</li>
</ol>
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<p>The <a href="../api_reference/C/envset_lk_max_locks.html" class="olink">DB_ENV->set_lk_max_locks()</a>, <a href="../api_reference/C/envset_lk_max_lockers.html" class="olink">DB_ENV->set_lk_max_lockers()</a>, and
and <a href="../api_reference/C/envset_lk_max_objects.html" class="olink">DB_ENV->set_lk_max_objects()</a> methods specify the maximum number of
locks, lockers, and locked objects supported by the lock subsystem,
respectively. The maximum number of locks is the number of locks that
can be simultaneously requested in the system. The maximum number of
lockers is the number of lockers that can simultaneously request locks
in the system. The maximum number of lock objects is the number of
objects that can simultaneously be locked in the system. Selecting
appropriate values requires an understanding of your application and its
databases. If the values are too small, requests for locks in an
application will fail. If the values are too large, the locking
subsystem will consume more resources than is necessary. It is better
to err in the direction of allocating too many locks, lockers, and
objects because increasing the number of locks does not require large
amounts of additional resources. The default values are 1000 of
each type of object.</p>
<p>The <a href="../api_reference/C/envset_lk_partitions.html" class="olink">DB_ENV->set_lk_partitions()</a> method specifies the number of lock
table partitions. Each partition may be accessed independently by
a thread and more partitions can lead to higher levels of concurrency.
The default is to set the number of partitions to be 10 times the number
of cpus that the operating system reports at the time the environment is
created. Having more than one partition when there is only one cpu is not beneficial and
the locking system is more efficient when there is a single partition.
Operating systems (Linux, Solaris) may report thread contexts as cpus and it
may be necessary to override the default to force a single partition on
a single hyperthreaded cpu system.
Objects and locks are divided among the partitions so it best to allocate
several locks and objects per partition. The system will force there
to be at least one per partition.
If a partition runs out of locks or objects it will steal what is needed
from the other partitions. This operation could impact performance if
it occurs too often.</p>
<p>When configuring a Berkeley DB Concurrent Data Store application, the number of lock objects needed
is two per open database (one for the database lock, and one for the
cursor lock when the <a href="../api_reference/C/envset_flags.html#set_flags_DB_CDB_ALLDB" class="olink">DB_CDB_ALLDB</a> option is not specified). The
number of locks needed is one per open database handle plus one per
simultaneous cursor or non-cursor operation.</p>
<p>Configuring a Berkeley DB Transactional Data Store application is more complicated. The recommended
algorithm for selecting the maximum number of locks, lockers, and lock
objects is to run the application under stressful conditions and then
review the lock system's statistics to determine the maximum number of
locks, lockers, and lock objects that were used. Then, double these
values for safety. However, in some large applications, finer
granularity of control is necessary in order to minimize the size of the
Lock subsystem.</p>
<p>The maximum number of lockers can be estimated as follows:</p>
<div class="itemizedlist">
<ul type="disc">
<li>If the database environment is using transactions, the maximum number
of lockers can be estimated by adding the number of simultaneously
active non-transactional cursors open database handles to the number of
simultaneously active transactions and child transactions (where a child
transaction is active until it commits or aborts, not until its parent
commits or aborts).</li>
<li>If the database environment is not using transactions, the maximum
number of lockers can be estimated by adding the number of
simultaneously active non-transactional cursors and open database
handles to the number of simultaneous non-cursor operations.</li>
</ul>
</div>
<p>The maximum number of lock objects needed for a transaction
can be estimated as follows:</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<p>
For each access to a non-Queue database, one lock object is
needed for each page that is read or updated.
</p>
</li>
<li>
<p>
For the Queue access method you will need one lock object per
record that is read or updated. Deleted records skipped by a
DB_NEXT or DB_PREV operation do not require a separate lock
object.
</p>
</li>
<li>
<p>
For Btree and Recno databases additional lock objects may be
needed for each node in the btree that has to be split due to
an update.
</p>
</li>
<li>
<p>
For Hash and Queue databases, every access must obtain a lock on
the metadata page for the duration of the access. This is not
held to the end of the transaction.
</p>
</li>
<li>
<p>
If the transaction performs an update that needs to allocate a page
to the database then a lock object for the metadata page will
be needed to the end of the transaction.
</p>
</li>
</ul>
</div>
<p>
Note that transactions accumulate locks over the transaction lifetime,
and the lock objects required by a single transaction is the total lock
objects required by all of the database operations in the transaction.
However, a database page (or record, in the case of the Queue access
method), that is accessed multiple times within a transaction only
requires a single lock object for the entire transaction. So if a
transaction in your application typically accesses 10 records, that
transaction will require about 10 lock objects (it may be a few more if
it splits btree nodes). If you have up to 10 concurrent threads in your
application, then you need to configure your system to have about 100
lock objects. It is always better to configure more than you need so
that you don't run out of lock objects. The memory overhead of
over-allocating lock objects is minimal as they are small structures.
</p>
<p>
The maximum number of locks required by an application cannot be easily
estimated. It is possible to calculate a maximum number of locks by
multiplying the maximum number of lockers, times the maximum number of
lock objects, times two (two for the two possible lock modes for each
object, read and write). However, this is a pessimal value, and real
applications are unlikely to actually need that many locks. Reviewing
the Lock subsystem statistics is the best way to determine this value.
</p>
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