Golang / GoLang Concurrency Mastery Interview Questions

Explain Go's GMP scheduler model. What are M, P, and G and how do they interact?

Go uses an M:N scheduler — M goroutines multiplexed onto N OS threads, managed by the Go runtime. The three key entities are:

GMP Entities
Entity	Symbol	Description
Goroutine	G	Lightweight user-space thread with its own 2 KB stack. Contains the goroutine's code, stack pointer, and scheduling state.
Machine (OS thread)	M	A real kernel thread that executes Go code. The number of active Ms equals GOMAXPROCS by default.
Processor (logical CPU)	P	A scheduling context holding a local run queue of runnable Gs. One P per logical CPU (controlled by GOMAXPROCS). An M must hold a P to execute Go code.

import "runtime"

// GOMAXPROCS sets the number of Ps (default = runtime.NumCPU())
runtime.GOMAXPROCS(4) // 4 Ps → up to 4 goroutines running in true parallel

// Query without changing
numP := runtime.GOMAXPROCS(0)
fmt.Println("Ps:", numP)

// Each P holds a local run queue: a lock-free ring buffer of ~256 G slots
// There is also a global run queue for overflow

// Work stealing: when a P's local queue is empty,
// it steals half the Gs from another P's queue
// This keeps all CPUs busy without a central scheduling lock

Goroutine lifecycle through GMP: (1) go func() creates a G and places it in the current P's local run queue. (2) The M bound to that P picks up the G and executes it. (3) If the G makes a blocking syscall, the M detaches from its P; the P binds to a new M and continues running other Gs. When the syscall returns, the original M tries to reacquire a P — if none is free, the G goes to the global queue and M parks. (4) Asynchronous preemption (Go 1.14+): a goroutine running too long without a function call receives a SIGURG signal and is preempted.

What does a P (Processor) in Go's GMP model hold?A physical CPU core allocation from the OS

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A local run queue of goroutines and a scheduling context — an M must hold a P to execute Go code

✓ Correct! Well done.

The goroutine's stack memory

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A pool of OS threads waiting for work

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What happens to the P when a goroutine makes a blocking syscall?The P waits for the syscall to complete before running other goroutines

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The P detaches from its blocked M, binds to a new M, and continues executing other goroutines

✓ Correct! Well done.

The P is destroyed and a new one is created when the syscall returns

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All other goroutines on the P pause until the syscall completes

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Golang / GoLang Concurrency Mastery Interview Questions

Explain Go's GMP scheduler model. What are M, P, and G and how do they interact?

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