Pi3 out of swap at < 50%

[Lyndon Nerenberg]

> I suspect your swap devices are too slow to keep up with instantaneous
> demands. ?An sdcard can easily have read and write latencies as big as
> 30 seconds; they make pretty poor swap devices (have gstat running in
> another window during the build to see what I mean, look at the ms/r
> and ms/w columns). ?USB flash drives may not be much better.

If it's an option, mount your swap partition over NFS from another server, 
and configure the mount settings to be as asynchronous as possible.