Performance recommendations

Moodle can be made to perform very well, at small usage levels or scaling up to many thousands of users. The factors involved in performance are basically the same as for any PHP-based database-driven system, and Moodle's design (with clear separation of application layers) allows for strongly scalable setups. (Please check the list of large Moodle installations.)

Large sites usually separate the web server and database onto separate servers, although for smaller installations this is typically not necessary.

It is possible to load-balance a Moodle installation, for example by using more than one webserver. The separate webservers should query the same database and refer to the same filestore area, but otherwise the separation of the application layers is complete enough to make this kind of clustering feasible. Similarly, the database could be a cluster of servers (e.g. a MySQL cluster), but this is not an easy task and you should seek expert support, e.g. from a Moodle Partner.

Obtain a baseline benchmark

Before attempting any optimization, you should obtain a baseline benchmark of the component of the system you are trying to improve. For Linux try LBS and for Windows use the Performance Monitor. The overall aim of adjustments to improve performance is to use RAM (cacheing) and to reduce disk-based activity. It is especially important to try to reduce swap file usage as much as you can.

Once you have quantitative data about how your system is performing currently, you'll be able to determine if the change you have made as has any real impact.

Hardware configuration

Note: The fastest and most effective change that you can make to improve performance is to increase the amount of RAM on your web server - get as much as possible (eg 4GB). Increasing primary memory will reduce the need for processes to swap to disk and will enable your server to handle more users.

Better performance is gained by obtaining the best processor capability you can, i.e. dual or dual core processors. A modern BIOS should allow you to enable hyperthreading, but check if this makes a difference to the overall performance of the processors by using a CPU benchmarking tool.
If you can afford them, use SCSI hard disks instead of SATA drives. SATA drives will increase your system's CPU utilization, whereas SCSI drives have their own integrated processors and come into their own when you have multiple drives. If you must have SATA drives, check that your motherboard and the drives themselves support NCQ (Native Command Queuing).
Size your swap file correctly. The general advice is to set it to 4 x physical RAM.
Use a RAID disk system. Install a hardware RAID controller (if you can) and install as follows:
- the operating system and swap drive on one set of disks configured as RAID-1.
- Moodle, Web server and Database server on another set of disks configured as RAID-5.
Check your own OS and vendor specific instructions for optimization steps.
- For Linux look at the Linux Performance Team site.
- For Windows set the sever to be optimized for network applications (Control Panel, Network Connections, LAN connection, Properties, File & Printer Sharing for Microsoft Networks, Properties, Optimization). You can also search the Microsoft TechNet site for optimization documents.

Web server performance

Linux or Unix is recommended for the server operating system. *nix operating systems generally require less memory than Mac OS X or Windows servers for doing the same task as the server is configured with just a shell interface. Additionally Linux does not have licensing fees attached, but can have a big learning curve if you're used to another operating system. If you have a large number of processors running SMP, you may also want to consider using a highly tuned OS such as Solaris.
You are strongly recommended to use a PHP accelerator to ease CPU load, such as APC (recommended), PHPA, Xcache or eAccelerator. (Take care to choose a PHP accelerator that is known to work well with your version of PHP and note that Turck MMCache is no longer maintained and can cause failures with PHP 5).
Performance of PHP is better when installed as an Apache/ISAPI module (rather than a CGI).
Consider reducing the number of modules that Apache loads in the httpd.conf file to the minumum necessary to reduce the memory needed. Also, use the latest version of Apache 2, which reduces memory usage further.
Alternatively, you can increase web server performance by using the light-weight webserver lighttpd in combination with PHP in fastCGI-mode instead of Apache. Lighttpd has a lower memory consumption than Apache. One single apache process requires more RAM than the whole lighttpd with all of its fastCGI-processes together. Note that Lighttpd is relatively difficult to configure and administration takes a more time.
For Unix/Linux systems, consider lowering MaxRequestsPerChild in httpd.conf to as low as 20-30 (if you set it any lower the overhead of forking begins to outweigh the benefits).
For a heavily loaded server, consider lowering the KeepAliveTimeout (Apache) or ListenBackLog (IIS - registry location is HKLM\ SYSTEM\ CurrentControlSet\ Services\ Inetinfo\ Parameters) to between 2 to 5. The default is 15 (seconds) - the higher the value the more server processes will be kept waiting for possibly idle connections.
Also check the memory_limit in php.ini, reduce it to 16M for Moodle version earlier than 1.7 (See this forum discussion). For Moodle 1.7 or later, it is recommended that the value of memory_limit should be 32M.
Here are some more IIS optimizations which alter this location in the registry:

HKLM\SYSTEM\CurrentControlSet\Services\Inetinfo\Parameters\

Change the MemCacheSize value to adjust the amount of memory (Mb) that IIS will use for its file cache (50% of available memory by default).
Change the MaxCachedFileSize to adjust the maximum size of a file cached in the file cache in bytes. Default is 262,144 (256K).
Create a new DWORD called ObjectCacheTTL to change the length of time (in milliseconds) that objects in the cache are held in memory. Default is 30,000 milliseconds (30 seconds).

Database performance

The following are MySQL specific settings which can be adjusted for better performance in your my.cnf (my.ini in Windows). To see the current values use the SHOW STATUS; or SHOW VARIABLES; commands. Important: You must make backups of your database before attempting to change any MySQL server configuration. After any change to the my.cnf, restart mysqld.

Enable the query cache with query_cache_type = 1. For most Moodle installs, set the query_cache_size to 36M and query_cache_min_res_unit to 2K. The query cache will improve performance if you are doing few updates on the database.
Set the table cache correctly. For Moodle 1.6 set this as table_cache = 159. Note that this figure will change depending on the number of modules and plugins you have installed. Find the number for your server by executing the mysql statement below. Look at the number of rows printed and set table_cache to this value.

SELECT table_name FROM information_schema.tables WHERE table_schema='yourmoodledbname';

Set the thread cache correctly. Adjust the value so that your thread cache utilization is as close to 100% as possible by this formula:

thread cache utilization (%) = (threads_created / connections) * 100

The key buffer can improve the access speed to Moodle's SELECT queries. The correct size depends on the size of the index files (.myi) and in Moodle 1.6 (without any additional modules and plugins), the recommendation for this value is key_buffer_size = 32M. Ideally you want the database to be reading once from the disk for every 100 requests so monitor that the value is suitable for your install by adjusting the value of key_buffer_size so that the following formulas are true:

key_read / key_read_requests < 0.01
key_write / key_write_requests <= 1.0

Set the maximum number of connections so that your users will not see a "Too many connections" message. Be careful that this may have an impact on the total memory used. MySQL connections usually last for milliseconds, so it is unusual even for a heavily loaded server fo this value to be over 200.
Manage high burst activity. If your Moodle install uses a lot of quizzes and you are experiencing performance problems (check by monitoring the value of threads_connected - it should not be rising) consider increasing the value of back_log.
Optimize your tables after data deletion. Backup your database first and then use:

OPTIMIZE table mdl_tablename;

Reduce the number of temporary tables saved to disk. Check this with the created_tmp_disk_tables value. If this is relatively large (>5%) increase tmp_table_size until you see a reduction. Note that this will have an impact on RAM usage.

Moodle Admin settings

Enable the language cache.
Large log files can cause overall performance to degrade over time. If you observe that the site has gradually got slower loading pages in the browser, reduce your Log life time setting (Admin/Configuration/Variables/Maintenance).
In Moodle 1.7 or later, enable the record cache. This will act as a primary cache and improve database access performance.
Performance can be greatly improved by allowing Moodle to use the system zip/unzip commands (rather than PHP-based zip libraries) - visit Admin/Configure/Variables and enter the path to the relevant executables. (Similarly, filling in the path to du will improve Moodle's speed at listing directory contents.)
Note that using secure web connections (https rather than http) carries a higher processing burden, both for the webserver and the client - particularly because cacheing cannot be used as effectively, so the number of file requests is likely to increase dramatically. For this reason using https for all Moodle pages is not recommended. You can enable https just for the login screen, simply from Moodle's config page.
Check your filters. Having too many filters active can have serious effects on server load, especially on lower-end systems. The number of active filters has a direct effect on the perceived latency of your site; that is the time taken for each page impression.
Enable the text cache but do not "Filter all strings" unless you have a specific need. If in doubt profile the performance, and see how your changes affect the processing time.
Check your anti-virus measures on the server. Although they are useful for preventing security holes being exploited, some "On-Demand" scanners can affect performance by scanning page content (word, ppt files etc).

Performance of different Moodle modules

Moodle's activity modules, filters, and other plugins can be activated/deactivated. If necessary, you may wish to deactivate some features (such as chat) if not required - but this isn't necessary. Some notes on the performance of certain modules:

The Chat module is said to be a hog in terms of frequent HTTP requests to the main server. This can be reduced by setting the module to use Streamed updates, or, if you're using a Unix-based webserver, by running the chat in daemon mode.
The quiz activity is known to stretch database performance. Try to optimise your database server by tuning. See for a brief report on performance for 55 students simultaneously using quizzes
The Moodle Cron task is triggered by calling the script cron.php. If this is called over HTTP (e.g. using wget or curl) it can take a large amount of memory on large installations. If it is called by directly invoking the php command (e.g. php -f /path/to/moodle/directory/admin/cron.php) efficiency can be much improved.

Documentation