Memcached is a simple key/value memory store that allows values from pretty much any application to be stored in memory for quick retrieval. Most languages have simple APIs that make this very easy from a programming standpoint. This also means that it’s not quite as simple to configure as APC (but similar for modern applications).

Installation and Configuration

To install memcached I recommend using your choice of installation (mine being emerge) and then checking the configuration (again in the case of Gentoo) located at /etc/conf.d/memcached.

Update the file to listen on the interface you want, use the memory you want, etc and then start the daemon up. Now if you’re using memcached on the same server as your apache server most webapps will be able to just turn on memcached and connect and work, but if you’re using memcached and apache on separate servers it’s a little more work.

WordPress and memcached

I’ve already touted the wordpress plugin: W3 Total Cache but unfortunately this plugin doesn’t allow you to easily configure memcached usage through the web interface. It defaults to using the memcached server located at 127.0.0.1:11211 which if we’re using separate servers is less than ideal. The file we need to modify with a simply sed (or by hand if you prefer knowing what’s going on) is %{DOCROOT}/wp-content/w3-total-cache-config.php.

We just need to change the memcached servers to point at our server’s location before enabling it in the interface. To easily accomplish this we can use sed:

sed -i -e ‘s/127.0.0.1:11211/: /g’ %{DOCROOT}/wp-content/w3-total-cache-config.php

Mediawiki and memcached

The other application we can easily setup for memcached is mediawiki. Mediawiki wants us to modify its LocalSettings.php with the following additions:

## Shared memory settings
$wgMainCacheType = CACHE_MEMCACHED;
$wgParserCacheType = CACHE_MEMCACHED;
$wgMessageCacheType = CACHE_MEMCACHED;
$wgMemCachedServers = array(“giskard.alunduil.com:11211″);

$wgSessionsInMemcached = true;

These settings simply turn on memcached (CACHE_MEMCACHED) for the various caching areas that mediawiki uses and specifies an array of memcached servers to store that caching information in.

If you do have more than one memcached server configured and available you can use the following syntax to include all of them with various weights:

$wgMemCachedServers = array(array(“host:port”, weight), array(“host2:port2″, weight));

Conclusion

Using memcached can greatly improve the performance of webapps or other applications that don’t need to present the most up to date information. This can reduce traffic to an overloaded database or configuration file or whatever other store you want to place a cache in front of.

Making PHP run faster is usually pretty easy when you see some of the code that people can write (myself included) but what if we either don’t want to look at their code or don’t want to fix the code? What else can we do to improve the runtime and the load time of PHP web sites (this website included)?

We’ll cover how to use APC (Alternative PHP Cache) to help alleviate some of the issues of a slow web site. Just like last time we optimized MySQL and talked about optimizing Apache we’ll continue on the path to getting PHP applications as fast as we can (without touching a line of code).

Enter APC

APC is PEAR project and a PHP module that after installed and a restart of Apache is already doing some work to make life better. By default APC turns on op-code caching which saves on the compile and execution times of PHP. The configuration for this module is pretty straight forward, but let’s look at a copy of the one I’m using:

; /etc/php/apache2-php5/ext-active/apc.ini (On Gentoo anyways …)
extension=apc.so
apc.enabled=”1″
apc.shm_segments=”4″
apc.shm_size=”128″
apc.num_files_hint=”1024″
apc.ttl=”7200″
apc.user_ttl=”7200″
apc.gc_ttl=”3600″
apc.cache_by_default=”1″
;apc.filters=”"
;apc.mmap_file_mask=”/tmp/apcphp5.XXXXXX”
apc.slam_defense=”0″
apc.file_update_protection=”2″
apc.enable_cli=”0″
apc.max_file_size=”1M”
apc.stat=”1″
apc.write_lock=”1″
apc.report_autofilter=”0″
apc.include_once_override=”0″
apc.rfc1867=”0″
apc.rfc1867_prefix=”upload_”
apc.rfc1867_name=”APC_UPLOAD_PROGRESS”
apc.rfc1867_freq=”0″
apc.localcache=”0″
apc.localcache.size=”512″
apc.coredump_unmap=”0″

Zooming In

These options are documented fairly thoroughly in the APC PHP Manual.

Most of these options can stay at their defaults and provide a pleasant experience but if we have the memory we should probably tweak these to get more out of them:

• apc.shm_segments – The number of chunks to use from /dev/shm.
• apc.shm_size – The size of aforementioned chunks.

These two parameters are the heart of APC and dictate the memory usage you’ll get out of it. If you take segments*size you’ll get the maximum amount of shm space used by APC for the cache. A limitation on shm_size is usually in place through the kernel and to determine what this is you can simply `cat /proc/sys/kernel/shmmax` (remember this command prints out B and APC expects MB).

Further Caching

APC also has an object cache available which can be used for just about anything but the application has to be modified to support this. If you’re using WordPress to host your site an excellent plugin for doing this is W3 Total Cache.

Conclusion

APC can help you get an edge out of your server but, without careful tuning, won’t get you more than the last inch of performance. Play with the memory settings until it isn’t over-utilizing memory (have a larger cache than your code base needs) or under-utilizing memory (having a smaller cache than your code base needs) but don’t forget you don’t want it to run into your swap space.