I have centos 6.4 server running purely mysqld db only where the current %MEM is 25.8 but I notice there is some swap in my top command used 6620K used. My total memory is 7990916K used is showing as 7427304K. I know not all memory is used because linux will normally take more than its required but its swapping now is worrying me. Anything I should do ?

Why does swap usage worry you? Since you have the resource, shouldn't you use it for something if necessary?
You do realize how incredibly tiny in the scheme of things 6620K is, right? You could probably just round that number down to zero in your head if it makes you less worried.

Dear Snowpine,
When should I get worried ? Yes I did even the vmstat nothing is showing up in the S1 and SO. Acctually why does linux take up practically everything of my ram ? Another thing why does mysql memory keep increasing? Will it increase till the threshold innodb_buffer_pool?

There is some good info here: http://www.linuxatemyram.com/

Also an interesting read: https://en.wikipedia.org/wiki/Swappiness

im guessing you are sitting at roughly 8G RAM in your server. if you are just now starting to touch swap, its nothing to worry about, but do keep an eye on it. if you start touching more and more swap you will want to increase the physical RAM in the server. while swap is nice to have, it is considerably slower in performance to physical RAM even with newer SSD drives.

Today its nothing to stress over. It should be monitored though. I would be concerned if you were using 95-100% physical RAM and 1G or more of swap. at that point, you need more physical RAM or you will start to see performance issues in the DB.