It's so strange.
outside the container, top shows all memory is in use (and some KBs of swap). ps auxww gave me a looong list of 0.0% MEM use processes (only lighttpd, mysql, and named show 0.1 to 0.4)
inside VM (vzctl enter VMID) top show 68MB use, and ps auxww, a lot of 0.0 (only the same...