I have a couple systems here which are swapping out constantly when the system is under load. This is almost certainly cache pressure since "under load" means "streaming large amounts of data to disk".
The odd thing is they almost never swap back in. The swap device is getting about 2MB/s worth of writes, and yet there's never more than a few kilobytes of swap in use. Examining it with atop, none of the processes on the system show any swap in use at all. Setting swappiness to 0 doesn't slow the rate down at all either, which is also interesting. I'm hoping somebody has some clues about how to figure out what's being swapped out and why it's being immediately deallocated/overwritten. Because it seems like this is just wear and tear with no useful purpose being served and I'd kind of like to make it stop, but would rather not do that by disabling swap entirely. LMP
