I have a machine with 64GB of memory in our lab. We have memory demanding processes and natually swap gets used. However when these processes are gone and the system is idle, we still see swap being used. Is this normal behavior?

Perfectly normal. If the pages that have been swapped out are not used then the system won't bother paging them back in again; keeps even more memory free for other uses.

For example, my small 360Mb machine:

% free total used free shared buffers cached Mem: 368844 330740 38104 0 14928 248348 -/+ buffers/cache: 67464 301380 Swap: 263160 560 262600

560K of swap in use even though there's 300Mb of free space; that 560K is just not in active use, so is better off on disk than in RAM.

If you _really_ cared you could swapoff to force the pages back to RAM and then swapon again. But it's not worth it.