MediaWiki uses too much RAM and CPU
The file cache only works for users who aren't logged in (try logging out). That's only one layer of caching, though: you should implement as many of the suggestions on the performance tuning page as you can. A PHP cache is particularly important.
now i know what my problem is exactly. the media wiki creates 40-50 idle mysql connections. the ram (now 1GB) increases until 999mb is reached. than the mysql service freezes.
There shouldn't be "idle" connections (although I guess there could be if you have persistent connections enabled). The various caching layers will reduce the number of MySQL threads that are needed. You should also configure MySQL's max_connections setting and Apache's MaxClients setting so that it won't use more memory than you have.
