What does runtime.Gosched do and when (if ever) should you use it?

runtime.Gosched() gives up the processor. The current G goes to the global run queue as runnable, and the P schedules something else. It does not sleep or block, and the goroutine continues later without anything needing to wake it.

for !done.Load() {
	runtime.Gosched() // polite spin — still a smell
}

Legitimate uses are rare:

• A cooperative yield inside a long CPU loop on Go versions before 1.14.
• Low-level lock-free code that briefly spins before parking.
• Making tests and demos show interleaving.

Since async preemption, it is almost never needed for fairness.

Gotchas:

• Gosched is not synchronization. Code that relies on it to order events is racy, and -race will say so.
• A spin-wait with Gosched still burns CPU. Use channels, sync.Cond or WaitGroup instead.
• Compare it with time.Sleep(0), which returns immediately, and runtime.Goexit(), which ends the goroutine after running its deferred calls.