Swap will be used when your RAM is all consumed. If you never reach your RAM limits your swap remains untouched, No worries its very normal. Maybe when you grow traffic swap will be used and will be highly helpful.
The problem is I have checked my other servers and all of them are using SWAP space and those have enough memory too. The customer is complaining his server is slow. I just wondered that, is this anything to do with slowness?
There's plenty else to slow the server down. Is mySQL and PHP cached? is the site making a massive amount of HD access? As far as performances go, swap slow down things, it would be a problem if you're using a massive amount of swap. Your memory management is as optimized as possible if everything's in RAM.
Sarcasm, the 6th Ponyman of the Apocalypse.
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Actually Linux will go into swap before all RAM is fully consumed. Usually after your RAM usage is >30ish % (determined by swapiness parameter), Linux may decide to swap out unfrequently used pages in favour for file cache to improve the general performance of the system. If your swappiness value is too low, then the swapping does not occur as frequently.
swappiness = 0 gives you the behaviour of trying not to swap unless necessary, such as being out of physical memory (RAM)
swappiness = 100 tells the kernel swap whenever the kernel feels like it
swappiness = 50 gives you the in-between behaviour.
In short, the higher the swappiness, the more it will swap.