vm.swappiness is a tunable kernel parameter that controls how much the kernel favors swap over RAM. At the source code level, it’s also defined as the tendency to steal mapped memory. A high swappiness value means that the kernel will be more apt to unmap mapped pages. A low swappiness value means the opposite, the kernel will be less apt to unmap mapped pages. In other words, the higher the vm.swappiness value, the more the system will swap.
More to read : http://www.linuxvox.com/2009/10/what-is-the-linux-kernel-parameter-vm-swappiness/
The following is the test. It seems that the higher swapiness the slower i/o rate. More to read : http://lwn.net/Articles/100978/
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