system uses too much memory

[kilburn]

Now repeat with me: free ram is not useful for anything, so I'm happy that the kernel uses it for caching and buffering purposes.

hint: read and understand the "-/+ buffers/cache" line, where it says that you actually have 3362mb of free ram that is being used by the kernel for buffering and/or caching purposes. If/when some program needs it, the kernel will happily handle it the necessary amount of ram.

It's standard linux behaviour regarding memory usage, and it's actually GOOD for you.