Explain the Go Scheduler: G, M, P Model

Problem Statement

In a Principal Engineer interview at Google, you're asked: "Explain how Go can run millions of goroutines on a few OS threads. What is the GMP model and how does work stealing improve performance?"

The GMP Model

G (Goroutine): User-space thread, starts at 2KB stack, grows dynamically
M (Machine): OS thread, executes goroutines
P (Processor): Logical processor, holds the run queue

Visual Representation

Key Scheduler Events

Event	What Happens
Goroutine blocks (I/O, channel)	M releases P, P picks another G
Goroutine syscall	M blocks with G, new M takes P
P run queue empty	Work stealing from other Ps
Goroutine runs too long (>10ms)	Preempted, put back in queue

Debugging with GODEBUG

Work Stealing Algorithm

GOMAXPROCS

Follow-up Questions

What happens when a goroutine makes a blocking syscall?
How does Go 1.14+ achieve asynchronous preemption?
Why doesn't increasing GOMAXPROCS beyond NumCPU help CPU-bound work?

Problem Statement

In a Principal Engineer interview at Google, you're asked: "Explain how Go can run millions of goroutines on a few OS threads. What is the GMP model and how does work stealing improve performance?"

The GMP Model

G (Goroutine): User-space thread, starts at 2KB stack, grows dynamically
M (Machine): OS thread, executes goroutines
P (Processor): Logical processor, holds the run queue

Visual Representation

Key Scheduler Events

Event	What Happens
Goroutine blocks (I/O, channel)	M releases P, P picks another G
Goroutine syscall	M blocks with G, new M takes P
P run queue empty	Work stealing from other Ps
Goroutine runs too long (>10ms)	Preempted, put back in queue

Debugging with GODEBUG

Work Stealing Algorithm

GOMAXPROCS

Follow-up Questions

What happens when a goroutine makes a blocking syscall?
How does Go 1.14+ achieve asynchronous preemption?
Why doesn't increasing GOMAXPROCS beyond NumCPU help CPU-bound work?

┌─────────────────────────────────────────────────────────┐ │ Go Runtime │ ├─────────────────────────────────────────────────────────┤ │ │ │ ┌─────┐ ┌─────┐ ┌─────┐ ┌─────┐ │ │ │ G │ │ G │ │ G │ │ G │ Global Run Q │ │ └──┬──┘ └──┬──┘ └──┬──┘ └──┬──┘ │ │ │ │ │ │ │ │ ┌──▼───────▼───────▼───────▼──┐ │ │ │ P (Processor) │ │ │ │ ┌───┐ ┌───┐ ┌───┐ ┌───┐ │ Local Run Queue │ │ │ │ G │ │ G │ │ G │ │ G │ │ │ │ │ └───┘ └───┘ └───┘ └───┘ │ │ │ └──────────────┬──────────────┘ │ │ │ │ │ ┌──────────────▼──────────────┐ │ │ │ M (OS Thread) │ │ │ │ Executing Goroutine │ │ │ └─────────────────────────────┘ │ │ │ └─────────────────────────────────────────────────────────┘