What is asynchronous preemption in Go (1.14+) and why was it introduced?

Before Go 1.14, goroutine scheduling was cooperative: a goroutine only yielded its processor at specific safe points — function call sites, channel operations, and syscalls. A CPU-bound goroutine in a tight loop with no function calls could starve other goroutines indefinitely and block GC stop-the-world phases.

// Pre-1.14: this goroutine could starve all others on its P indefinitely
go func() {
    for {
        x := 0
        for i := 0; i < 1_000_000_000; i++ { x++ }
        // No function calls → no scheduling point
        // Other goroutines on this P cannot run
        // GC STW cannot proceed — GC pause stretches indefinitely
    }
}()

// Go 1.14+: asynchronous preemption via SIGURG
// sysmon goroutine detects a goroutine running on a P for > 10ms
// It sends SIGURG to the OS thread running that goroutine
// The signal handler inserts a preemption point; the goroutine yields

// Effect:
// - Tight loops no longer starve other goroutines
// - GC STW completes in bounded time regardless of goroutine behaviour
// - Programs are more responsive under CPU-heavy workloads

// runtime.Gosched() — explicit cooperative yield (still useful)
for i := 0; i < 1_000_000; i++ {
    doHeavyChunk(i)
    if i%1000 == 0 {
        runtime.Gosched() // voluntarily yield every 1000 iterations
    }
}

// Preemption safety: goroutine stacks may be moved during preemption
// → All stack references must be valid Go pointers (enforces unsafe.Pointer rules)

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