Virtual memory is built into many operating systems. Since they each need it only when they are running, i.e., never at the same time, the swap spaces of all but the currently running one are being wasted. It would be more efficient for them to share a single swap space. This is possible, but can require a bit of hacking. The Tips-HOWTO at http://www.tldp.org/HOWTO/Tips-HOWTO.html, which contains some advice on how to implement this.