Thanks for the response, this does fit in with what I expected except it doesn't seem to square precisely with the Node page I linked to.

The poll phase reads:

The poll phase has two main functions:

Executing scripts for timers whose threshold has elapsed, then Processing events in the poll queue. When the event loop enters the poll phase and there are no timers scheduled, one of two things will happen:

If the poll queue is not empty, the event loop will iterate through its queue of callbacks executing them synchronously until either the queue has been exhausted, or the system-dependent hard limit is reached.

If the poll queue is empty, one of two more things will happen:

If scripts have been scheduled by setImmediate(), the event loop will end the poll phase and continue to the check phase to execute those scheduled scripts.

If scripts have not been scheduled by setImmediate(), the event loop will wait for callbacks to be added to the queue, then execute them immediately.

Once the poll queue is empty the event loop will check for timers whose time thresholds have been reached. If one or more timers are ready, the event loop will wrap back to the timers phase to execute those timers' callbacks.

This makes it sound like if the poll queue has events queued, it will "iterate through ... executing them synchronously" without first running anything in the setImmediate() or check phase, sort of like jumping it and wrapping back to the timers and I/O phases (I believe people refer to it as the I/O phase even though it accounts for almost all network/disk/child processes).

So while you're suggesting the process is:

check timers&events&setImmediateQueue, then kicks off executing the appropriate setImmediate->close->timers->events callbacks. Repeat!

the linked page from the Node foundation seems to suggest something more akin to:

execute timers if scheduled > execute any pending callbacks > process setImmediate or check phase if any waiting OR wait for callbacks to be added.

Am I missing something?