If the scheduler is doing _any_ sort of accounting at all to figure out any remote sort of fairness balancing at all, then whatever resolution that is probably works.

At least for Linux, offhand, popular task scheduler frequencies used to be 100 and 1000hz.

Looks like the Kernel's tracking that for tasks:

https://www.kernel.org/doc/html/latest/scheduler/sched-desig...

"In CFS the virtual runtime is expressed and tracked via the per-task p->se.vruntime (nanosec-unit) value."

I imagine the .vruntime struct field is still maintained with the newer "EEVDF Scheduler".

...

A Userspace task scheduler could similarly compare the DEADLINE against that runtime value. It would still reach that deadline after the minimum wait has passed, and thus be 'background GCed' at a time of the language's choice.

The issue is that no scheduler manages futures. The scheduler sees tasks, futures are just a struct. See discussion of embedded above: there is no “kernel esque” parallel thread

On Linux there is the VDSO, which on all mainstream architectures allows you to do `clock_gettime` without going through a syscall. It should take on the order of (double digit) nanoseconds.