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239 lines
11 KiB
HTML
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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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<html xmlns="http://www.w3.org/1999/xhtml">
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
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<title>Configuring locking: sizing the system</title>
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<link rel="stylesheet" href="gettingStarted.css" type="text/css" />
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<meta name="generator" content="DocBook XSL Stylesheets V1.73.2" />
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<link rel="start" href="index.html" title="Berkeley DB Programmer's Reference Guide" />
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<link rel="up" href="lock.html" title="Chapter 16. The Locking Subsystem" />
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<link rel="prev" href="lock_config.html" title="Configuring locking" />
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<link rel="next" href="lock_stdmode.html" title="Standard lock modes" />
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</head>
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<body>
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<div xmlns="" class="navheader">
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<div class="libver">
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<p>Library Version 11.2.5.3</p>
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</div>
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<table width="100%" summary="Navigation header">
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<tr>
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<th colspan="3" align="center">Configuring locking: sizing the system</th>
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</tr>
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<tr>
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<td width="20%" align="left"><a accesskey="p" href="lock_config.html">Prev</a> </td>
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<th width="60%" align="center">Chapter 16.
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The Locking Subsystem
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</th>
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<td width="20%" align="right"> <a accesskey="n" href="lock_stdmode.html">Next</a></td>
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</tr>
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</table>
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<hr />
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</div>
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<div class="sect1" lang="en" xml:lang="en">
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<div class="titlepage">
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<div>
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<div>
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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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</div>
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</div>
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</div>
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<p>
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The amount of memory available to the locking system is specified
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using the <a href="../api_reference/C/envset_memory_max.html" class="olink">DB_ENV->set_memory_max()</a> method. Sizing of the enviroment
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using the <a href="../api_reference/C/envset_memory_max.html" class="olink">DB_ENV->set_memory_max()</a> method is discussed in
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<a class="xref" href="env_size.html" title="Sizing a database environment">Sizing a database environment</a>. Here we will
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discuss how to estimate the number of objects your application is
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likely to lock. Since running out of memory for locking structures
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is a fatal error requiring reconfiguration and restarting the
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environment it is best to overestimate the numbers.
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</p>
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<p>
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When configuring a Berkeley DB Concurrent Data Store application,
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the number of lock objects needed is two per open database (one for
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the database lock, and one for the cursor lock when the
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<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
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needed is one per open database handle plus one per simultaneous
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cursor or non-cursor operation.
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</p>
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<p>
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Configuring a Berkeley DB Transactional Data Store application is
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more complicated. The recommended algorithm for selecting the
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number of locks, lockers, and lock objects is to run the
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application under stressful conditions and then review the lock
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system's statistics to determine the number of locks,
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lockers, and lock objects that were used. Then, double these
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values for safety. However, in some large applications, finer
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granularity of control is necessary in order to minimize the size
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of the Lock subsystem.
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</p>
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<p>
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The number of lockers can be estimated as follows:
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</p>
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<div class="itemizedlist">
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<ul type="disc">
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<li>
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If the database environment is using transactions, the
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number of lockers can be estimated by adding the number of
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simultaneously active non-transactional cursors and open database
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handles to the number of simultaneously active transactions and
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child transactions (where a child transaction is active until
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it commits or aborts, not until its parent commits or aborts).
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</li>
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<li>
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If the database environment is not using transactions, the
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number of lockers can be estimated by adding the number
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of simultaneously active non-transactional cursors and open
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database handles to the number of simultaneous non-cursor
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operations.
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</li>
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</ul>
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</div>
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<p>
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The number of lock objects needed for a transaction
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can be estimated as follows:
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</p>
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<div class="itemizedlist">
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<ul type="disc">
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<li>
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<p>
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For each access to a non-Queue database, one lock object is
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needed for each page that is read or updated.
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</p>
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</li>
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<li>
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<p>
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For the Queue access method you will need one lock object per
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record that is read or updated. Deleted records skipped by a
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DB_NEXT or DB_PREV operation do not require a separate lock
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object.
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</p>
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</li>
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<li>
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<p>
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For Btree and Recno databases additional lock objects may be
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needed for each node in the btree that has to be split due to
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an update.
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</p>
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</li>
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<li>
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<p>
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For Hash and Queue databases, every access must obtain a lock on
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the metadata page for the duration of the access. This is not
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held to the end of the transaction.
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</p>
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</li>
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<li>
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<p>
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If the transaction performs an update that needs to allocate a page
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to the database then a lock object for the metadata page will
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be needed to the end of the transaction.
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</p>
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</li>
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</ul>
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</div>
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<p>
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Note that transactions accumulate locks over the transaction lifetime,
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and the lock objects required by a single transaction is the total lock
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objects required by all of the database operations in the transaction.
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However, a database page (or record, in the case of the Queue access
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method), that is accessed multiple times within a transaction only
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requires a single lock object for the entire transaction. So if a
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transaction in your application typically accesses 10 records, that
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transaction will require about 10 lock objects (it may be a few more if
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it splits btree nodes). If you have up to 10 concurrent threads in your
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application, then you need to configure your system to have about 100
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lock objects. It is always better to configure more than you need so
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that you don't run out of lock objects. The memory overhead of
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over-allocating lock objects is minimal as they are small structures.
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</p>
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<p>
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The number of locks required by an application cannot be easily
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estimated. It is possible to calculate a number of locks by
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multiplying the number of lockers, times the 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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</p>
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<p>
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By default a minimum number of locking objects are allocated at
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startup. To avoid contention due to allocation the application may
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use the <a href="../api_reference/C/envset_memory_init.html" class="olink">DB_ENV->set_memory_init()</a> method to preallocate and initialize
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the following lock structures:
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</p>
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<div class="itemizedlist">
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<ul type="disc">
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<li>
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<p>
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<code class="literal">DB_MEM_LOCK</code>
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</p>
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<p>
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Specifies the number of locks that can be
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simultaneously requested in the system.
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</p>
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</li>
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<li>
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<p>
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<code class="literal">DB_MEM_LOCKER</code>
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</p>
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<p>
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Specifies the number of lockers that can
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simultaneously request locks in the system.
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</p>
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</li>
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<li>
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<p>
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<code class="literal">DB_MEM_LOCKOBJECTS</code>
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</p>
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<p>
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Specifies the number of objects that can
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simultaneously be locked in the system.
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</p>
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</li>
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</ul>
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</div>
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<p>
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In addition to the above structures, sizing your locking subsystem
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also requires specifying the number of lock table partitions. You
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do this using the <a href="../api_reference/C/envset_lk_partitions.html" class="olink">DB_ENV->set_lk_partitions()</a> method. Each partition
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may be accessed independently by a thread. More partitions can lead
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to higher levels of concurrency. The default is to set the number
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of partitions to be 10 times the number of cpus that the operating
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system reports at the time the environment is created. Having more
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than one partition when there is only one cpu is not beneficial
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because the locking system is more efficient when there is a single
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partition. Some operating systems (Linux, Solaris) may report
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thread contexts as cpus, and so it may be necessary to override the
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default to force a single partition on a single hyperthreaded cpu
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system. Objects and locks are divided among the partitions so it is
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best to allocate several locks and objects per partition. The
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system will force there to be at least one per partition. If a
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partition runs out of locks or objects it will steal what is needed
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from the other partitions. This operation could impact performance
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if it occurs too often. The final values specified for the locks
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and lock objects should be more than or equal to the number of lock
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table partitions.
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</p>
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</div>
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<div class="navfooter">
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<hr />
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<table width="100%" summary="Navigation footer">
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<tr>
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<td width="40%" align="left"><a accesskey="p" href="lock_config.html">Prev</a> </td>
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<td width="20%" align="center">
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<a accesskey="u" href="lock.html">Up</a>
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</td>
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<td width="40%" align="right"> <a accesskey="n" href="lock_stdmode.html">Next</a></td>
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</tr>
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<tr>
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<td width="40%" align="left" valign="top">Configuring locking </td>
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<td width="20%" align="center">
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<a accesskey="h" href="index.html">Home</a>
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</td>
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<td width="40%" align="right" valign="top"> Standard lock modes</td>
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</tr>
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</table>
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</div>
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</body>
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</html>
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