I've created PHP application for Facebook. It uses MySQL, Memcached and works on Lighttpd on Centos 2,6 Ghz and 2 GB RAM.
it's basically one PHP file that after first run is cached and every next time it's served in 0,8 ms directly from Memcached.
What more can I do to shorten this time?
Once you get to 0.8 ms, I'm not sure you can go any lower.
However, you can set up multiple servers to handle many concurrent requests (with a common memcached). You will then be able to scale very high by simply adding a new server when you reach limits.
Run XDebug and run the profiler. There you can see if you have any functions that take an abnormal length of time.
I recommend using a program that reads those logs. Like kcachegrind etc.
BTW, when you enter profiler country, there's no turning back.
You can put a http-proxy and load balance to multiple servers. Most http-proxies can double as a cache, which can take load off the application.
Also, make sure that you send the proper http headers, so that your pages are client-side cacheable. This may reduce the number of requests, as the clients will reuse their cached version.
Do you have a php script which gets data from memcached and displays them, or the web server itself gets data, directly from memcached?
I don't know if lighthttpd can display page directly from memcached, but i know nginx can do this google for ngx_http_memcached_module
