>lower than IDLE

Any idea why that's necessary ? So it's preempted by a IDLE process.

Windows NT thread scheduling does not work that way. If the Zero Page Thread is ready to run, the processor's Idle Thread is not scheduled. Idle Threads do not live on ready queues, and are only dispatched when there is no thread available from the processor's ready queue.

Also, do not conflate the user-thread idle priority class with the (non-)priorities of Idle Threads.

IDLE is the priority of a process that runs only when the system is otherwise idle. It shouldn't be preempted by zeroing memory.

But then under what circumstances will the zero page thread ever be scheduled?

It's confusing, but "idle" is a priority class as well as a state: https://docs.microsoft.com/en-us/windows/desktop/procthread/...

The idle task (when there's nothing to run) doesn't have a priority at all, it just gets run when nothing else is available. the "idle" priority is an actual priority, for when there's no higher-priority task to run. The ZPT is prioritized below "idle", so super-duper-low priority.

Thank you for this clear explanation. It is confusing!

When there are no runnable idle threads.

So the zero page process runs when one core is busy but the rest are free?

It runs when the there are no higher-priority processes to schedule on a particular core. IDLE processes are higher priority, so as long as an IDLE process can be scheduled, the zero page process won't run.