You reboot a machine every time it starts to use swap?? What the hell kind of applications are you running?
Even with java apps (which I grant you are difficult to estimate memory limits on without knowing the application's design) swapping can be a useful way to page out unneeded parts of memory - and more importantly, keep needed parts of memory intact. This can also mean keeping the Java processes in memory while paging out apps which are less crucial, which leaves more room for Java, etc.
Swap is, for lack of a better comparison, the canvas sheet you use to catch someone jumping off a building, or a new york city sewer. In the first example you can use it to save your applications/servers so you don't need to reboot them (the higher your availability requirements, the less you can stand random reboots). In the second example it's the place you send those inhabitants you don't deem worthy of RSS.
The other thing you have to consider is the idea of memory overcommit in application and kernel design. The system is built with a promise that it has way more memory than it actually physically does. Applications will always reserve a fake huge chunk of memory and doesn't care that the system is lying to them about how much is really available. Without swap, when these apps attempt to use the nonexistent available memory, they crash. With swap, they survive.
Then there's apps designed to rely on swap like the Varnish malloc() and file storage methods, or database servers. Even if you disable swap there are still performance and stability problems related to the VM, and understanding this helps your apps run more efficiently. (http://blog.jcole.us/2012/04/16/a-brief-update-on-numa-and-m...)
Even with java apps (which I grant you are difficult to estimate memory limits on without knowing the application's design) swapping can be a useful way to page out unneeded parts of memory - and more importantly, keep needed parts of memory intact. This can also mean keeping the Java processes in memory while paging out apps which are less crucial, which leaves more room for Java, etc.
Swap is, for lack of a better comparison, the canvas sheet you use to catch someone jumping off a building, or a new york city sewer. In the first example you can use it to save your applications/servers so you don't need to reboot them (the higher your availability requirements, the less you can stand random reboots). In the second example it's the place you send those inhabitants you don't deem worthy of RSS.
The other thing you have to consider is the idea of memory overcommit in application and kernel design. The system is built with a promise that it has way more memory than it actually physically does. Applications will always reserve a fake huge chunk of memory and doesn't care that the system is lying to them about how much is really available. Without swap, when these apps attempt to use the nonexistent available memory, they crash. With swap, they survive.
Then there's apps designed to rely on swap like the Varnish malloc() and file storage methods, or database servers. Even if you disable swap there are still performance and stability problems related to the VM, and understanding this helps your apps run more efficiently. (http://blog.jcole.us/2012/04/16/a-brief-update-on-numa-and-m...)