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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 amount of memory available to the locking system is
specified using the <a href="../api_reference/C/envset_memory_max.html" class="olink">DB_ENV->set_memory_max()</a> method. Sizing of the
enviroment using the <a href="../api_reference/C/envset_memory_max.html" class="olink">DB_ENV->set_memory_max()</a> method is discussed
in <a class="xref" href="env_size.html" title="Sizing a database environment">Sizing a database environment</a>. Here
we will discuss how to estimate the number of objects your
application is likely to lock. Since running out of memory for
locking structures is a fatal error requiring reconfiguration
and restarting the environment it is best to overestimate the
numbers.
</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 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 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 number of lockers can be estimated as follows:
</p>
<div class="itemizedlist">
<ul type="disc">
<li>
If the database environment is using transactions,
the number of lockers can be estimated by adding the
number of simultaneously active non-transactional cursors
and 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 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 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 number of locks required by an application cannot be
easily estimated. It is possible to calculate a number of
locks by multiplying the number of lockers, times the 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>
<p>
By default a minimum number of locking objects are
allocated at startup. To avoid contention due to allocation
the application may use the <a href="../api_reference/C/envset_memory_init.html" class="olink">DB_ENV->set_memory_init()</a> method to
preallocate and initialize the following lock structures:
</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<p>
<code class="literal">DB_MEM_LOCK</code>
</p>
<p>
Specifies the number of locks that can be
simultaneously requested in the system.
</p>
</li>
<li>
<p>
<code class="literal">DB_MEM_LOCKER</code>
</p>
<p>
Specifies the number of lockers that can
simultaneously request locks in the system.
</p>
</li>
<li>
<p>
<code class="literal">DB_MEM_LOCKOBJECTS</code>
</p>
<p>
Specifies the number of objects that can
simultaneously be locked in the system.
</p>
</li>
</ul>
</div>
<p>
In addition to the above structures, sizing your locking
subsystem also requires specifying the number of lock table
partitions. You do this using the <a href="../api_reference/C/envset_lk_partitions.html" class="olink">DB_ENV->set_lk_partitions()</a>
method. Each partition may be accessed independently by a
thread. 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 because the locking system is more efficient when
there is a single partition. Some operating systems (Linux,
Solaris) may report thread contexts as cpus, and so 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 is 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. The final values specified for the locks and
lock objects should be more than or equal to the number of
lock table partitions.
</p>
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