Drew Webber (mcdruid) is a UK Drupal developer, PHP programmer and linux sysadmin. This is his website.
There are several different PHP accelerators to choose from, but according to wikipedia "APC is quickly becoming the de-facto standard PHP caching mechanism as it will be included built-in to the core of PHP starting with PHP 6".
I recently put together a new development webserver in a virtualbox virtual machine, and as I was setting it up I thought I'd take the opportunity to test how much difference APC actually makes to a simple Drupal site.
I was using Ubuntu server. On newer releases APC is available from the package manager...
$ apt-cache search php-apc
php-apc - APC (Alternative PHP Cache) module for PHP 5
$ sudo apt-get install php-apc
...however I'm using Ubuntu 8.04 LTS (Hardy Heron) and there's no apc-php package. It's not hard to install via PECL / PEAR though. First some dependencies need to be installed, then PECL can be used to install APC.
$ sudo apt-get install php-pear php5-dev apache2-threaded-dev build-essential
$ sudo pecl install apc
This last command will produce a ton of output, but one of the last lines will tell you to add this to your php.ini file (which you'll find in /etc/php5/apache2/php.ini) - you'll probably have to do so manually.
Restart apache, and you should see a new APC section in phpinfo() which will confirm it's enabled. There's a small php script which gives you some useful info about APC, which you'll find in /usr/share/php/apc.php - you could use a symlink to allow you to get to this file in your browser to see the stats and graphs it produces to tell you what files it has cached, and info on cache hits and misses.
I've left the APC default settings - which in my case was only 30mb of memory being used for cache, and run some basic tests using Apache Bench on a simple Drupal 6 site. The actual performance figures are not that important (this is a virtual machine on my laptop, not a production server), but it's interesting to see how much difference it makes turning APC on.
I tested two pages - the very simple homepage, and another page which displays a relatively long webform. The AB command I used was for 100 requests with 10 concurrent requests. e.g.
$ ab -n 100 -c 10 http://mytestsite.example/webform/
|Test||Requests / Second|
|homepage (APC off)||2.78|
|webform (APC off)||1.72|
|homepage (APC on)||8.36|
|webform (APC on)||3.77|
|Test||Milliseconds / Request|
|homepage (APC off)||359.68|
|webform (APC off)||582.94|
|homepage (APC on)||119.61|
|webform (APC on)||265.41|
You can see that the effect of APC on the simple homepage is more dramatic than on the webform page. This is almost certainly because the database has to do a lot more work to build the latter, and APC's not going to help on that front. However, we can say on the simple page APC makes Drupal perform almost 3 times faster. With the more database-heavy webform page, the improvement is slightly less - but we're still looking at a doubling in performance.
This is obviously not a hugely detailed test, but it certainly leaves me in no doubt that installing APC represents a quick and easy way to achieve a huge improvement in performance for Drupal sites.