Running 5.6 update 6 here ... uptime 42 days and that after a reboot to get to update 6. Mr. Chan, please provide some more details as to your hardware/software configuration, whether this is a new or upgraded installation, which contribs you have installed, etc. As it stands you have not given nearly enough information.
As to seeing RAM used in phpSysInfo, as has been discussed many times in these forums, SME WILL use all available RAM, freeing what it's using for cache as necessary for system processes. Use SysMon to get more detailed info as to how RAM is being allocated.