Hello,

I have to daily reboot my VPS Quad Core with 4GB memory and 2GB Swap due to all memory is in use.

Can somebody tell me how I can see what is consuming that much memory ?

Linux will normally use 99% of it's available memory, this does refer to swap however, this is not an issue and by design.

Okay ... can you tell me how it comes my forum is slow ?

to understand linux memory usage read http://www.linuxatemyram.com/

Connect to the server via SSH and monitor "top" for a day and see what it freezes on the next time it crashes. If you are running out of RAM one of the last things you would probably see is "kswapd" which is the process that moves data back and forth from virtual memory. Your IOWait would also skyrocket at the end.

Would also be interesting to see what else is happening in "top" at the time of failure. Also run Mysqltuner to see if MySQL has more ram allocated to it then it should for the amount of RAM installed in the server. You always want to make sure that your services are configured in way where even if there is a problem they can not use more resoucres then the server has avalible to it so that 1 process such as MySQL can't down the whole machine.