Specifically, earlyoom’s README says it repeatedly checks (“periodically polls”) the memory pressure, using CPU each time even when there is no change. The “poll” system call waits for the kernel to notify the process that the file has changed, using no CPU until the call resolves. It’s unclear what systemd-oomd does, because it uses the phrase “periodically polls”,

The "poll" system call does not wait until a file changes.

s/the file has changed/it has published new data to the file descriptor/

See https://docs.kernel.org/accounting/psi.html

Poll takes a timeout parameter. ‘Not polling’ is just a really long timeout

"Let the underlying platform do the polling and return once the condition is met"