> When the notification for the I/O completion comes in (possibly by a
> queued sigevent, another POSIX API not yet NetBSD), the completion
> handler can then do a pthread_cond_signal() to wake up the thread
> waiting for that i/o completion, informing the userland scheduler that
> it's runnable again.

Scheduler activations is another way to implement this.
It doesn't use queued sigevent for notification, but use upcall.
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soda