Is there a way to tell Linux that it shouldn't swap out a particular processes' memory to disk?
Its a Java app, so ideally I'm hoping for a way to do this from the command line.
I'm aware that you can set the global swappiness to 0, but is this wise?
You can do that by the mlock family of syscalls. I'm not sure, however, if you can do it for a different process.
You can do this via the mlockall(2) system call under Linux; this will work for the whole process, but do read about the argument you need to pass.
Do you really need to pull the whole thing in-core? If it's a java app, you would presumably lock the whole JVM in-core. I don't know of a command-line method for doing this, but you could write a trivial program to call fork, call mlockall, then exec.
You might also look to see if one of the access pattern notifications in madvise(2) meets your needs. Advising the VM subsystem about a better paging strategy might work out better if it's applicable for you.
Note that a long time ago now under SunOS, there was a mechanism similar to madvise called vadvise.
As super user you can 'nice' it to the highest priority level -20 and hope that's enough to keep it from being swapped out. It usually is. Positive numbers lower scheduling priority. Normal users cannot nice upwards (negative nos.)
Except in extremely unusual circumstances, asking this question means that You're Doing It Wrong(tm).
Seriously, if Linux wants to swap and you're trying to keep your process in memory then you're putting an unreasonable demand on the OS. If your app is that important then 1) buy more memory, 2) remove other apps/daemons from the machine, or dedicate a machine to your app, and/or 3) invest in a really fast disk subsystem. These steps are reasonable for an important app. If you can't justify them, then you probably can't justify wiring memory and starving other processes either.
Why do you want to do this?
If you are trying to increase performance of this app then you are probably on the wrong track. The OS will swap out a process to increase memory for disk cache - even if there is free RAM, the kernel knows best (actauly the samrt guys that wrote the scheduler know best).
If you have a process that needs responsiveness (it's swapped out while not used and you need it to restart quickly) then nice it to high priority, mlock, or using a real time kernel might help.