What is the GMP model?

Question

Accepted Answer

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

**Components:**
- **G:** Execution unit with stack, PC, state.
- **M:** OS thread.
- **P:** Logical processor with local queue.

```go
fmt.Println("GOMAXPROCS:", runtime.GOMAXPROCS(0))
runtime.Gosched() // Yield
```

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

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

**When to tune:**
- CPU-bound: GOMAXPROCS = NumCPU.
- Containers: adjust for cgroup.

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

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