I just talked at the Forum PHP Paris about tuning and cache. The talk was titled “Optimisation d’un site web 2.0 : le cas wat.tv“.

Here are the slides (in french).

I’ve been playing with varnish for a few days now. Until now, I have been using lighttpd as an HTTP accelerator, in front of my apache servers.

I love lighttpd a lot : it is simple, very efficient, and quite customizable through lua. But it is not a great HTTP accelerator :

• it only uses non-persistent HTTP connections to the back-end,
• it does not cache (a simple caching reverse-proxy can be written is lua using mod_magnet, but this solution is not satisfactory).

I had to find a better alternative…

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A friend of mine pointed me to some nice tuning slides, written by Ilia Alshanetsky (PHP 5.2 release manager).

If you have already used apache on a medium/heavily loaded website, you already know the limitations of the pre-fork model traditionnaly used by apache. Each client connection uses a dedicated apache process, and a dedicated PHP interpreter, each process consuming a couple Megs (50-150MB of RAM for large PHP apps) of RAM.

When using KeepAlives (KeepAlive On), apache processes are mostly idle, waiting for the client to send another HTTP request. When not using KeepAlives (KeepAlive Off), page load times are slower, because each object requires a new TCP connection to be opened - and the server can be overloaded by the number of TCP open/closes.
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I have seen quite a few PHP apps with hundreds of defines. Those are generally loaded on each request, and when every tuning has already been done, those define can amount to a significant proportion of the PHP execution time. Here is a little benchmark :
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