The watermark is not some complex computed value, but actually the
statistic itself. We can get rid of handling a threshold in qemu by just
signalling "something has changed with this stat".
I'm really not arguing that qemu should do anything complex or even
define policy. It's just about avoiding polling all the time when
nothing has changed and polling too late when things are changing quickly.
Polling is really the right solution. It gives the management tool
ultimate flexibility in tweaking the heuristics as they see fit.
Isn't providing this flexibility completely orthogonal to polling vs.
event-based?