What is the GMP model?

Question

Accepted Answer

**GMP** — Goroutine (G), Machine (M), Processor (P). Go's scheduler for efficient concurrency.

**Components:**
- **G (Goroutine):** Execution unit with stack, PC, state.
- **M (Machine):** OS thread that executes goroutines.
- **P (Processor):** Logical processor with local run queue.

```go
// Check GOMAXPROCS
fmt.Println("GOMAXPROCS:", runtime.GOMAXPROCS(0))
fmt.Println("NumCPU:", runtime.NumCPU())

// Runtime hints
runtime.Gosched()  // Yield
runtime.LockOSThread() // Pin to thread
```

**Work stealing:**
1. P's queue empty → check global queue.
2. Global empty → steal from random P.
3. Steal half of victim's queue.
4. Nothing → check network poller.
5. Still nothing → M parks.

**Handoff:**
- M enters syscall → P detached → given to another M.

**When to tune:**
- CPU-bound: GOMAXPROCS = NumCPU.
- I/O-bound: higher GOMAXPROCS.
- Containers: adjust for cgroup limits.

**Common mistakes:**
- Not adjusting GOMAXPROCS in containers.
- Confusing `GOMAXPROCS(0)` (read) with setting.
- Assuming goroutines always run in parallel.
- Using `LockOSThread()` without need.
- Not understanding work stealing randomness.

Go: What is the GMP model?