How does Go's goroutine scheduler work? Explain the GMP model.

Go uses a cooperative/preemptive M:N scheduler — M goroutines multiplexed onto N OS threads, where N defaults to GOMAXPROCS (number of logical CPUs). The scheduler uses three key entities:

// GOMAXPROCS controls the number of P's (and thus OS threads active)
import "runtime"
runtime.GOMAXPROCS(4) // use 4 OS threads for parallel execution
fmt.Println(runtime.GOMAXPROCS(0)) // 0 = query without changing

// Scheduler events that cause a context switch:
// 1. Goroutine blocks on channel send/receive
// 2. Goroutine blocks on system call (M parks, P finds another M)
// 3. go statement (new G added to local run queue of current P)
// 4. runtime.Gosched() — voluntary yield
// 5. Function call (Go 1.14+: asynchronous preemption via signals)

// Work stealing: when P's local queue is empty,
// it steals half the goroutines from another P's queue

// Global run queue: accessed when local queue has > 256 Gs,
// or periodically to ensure fairness

// View scheduler decisions
// GOTRACE=scheduler ./myapp  — not a real flag, but:
// go tool trace trace.out    — after: f, _ := os.Create("trace.out")
//                              trace.Start(f); ...; trace.Stop()

System call handling: When a goroutine makes a blocking system call (e.g., reading a file), the M detaches from its P. The P then attaches to another idle M (or creates a new one), so other goroutines continue running. When the system call returns, the original M tries to reacquire a P; if none is available, it parks and the goroutine goes to the global queue.

Asynchronous preemption (Go 1.14+): the runtime sends SIGURG signals to goroutines that have run too long without a function call, forcing a context switch. This prevents a CPU-intensive goroutine from starving others even without cooperative yield points.

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