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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?

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
EntitySymbolDescription
GoroutineGLightweight user-space thread with its own 2 KB stack. Contains the goroutine's code, stack pointer, and scheduling state.
Machine (OS thread)MA real kernel thread that executes Go code. The number of active Ms equals GOMAXPROCS by default.
Processor (logical CPU)PA 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?
What happens to the P when a goroutine makes a blocking syscall?

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Explain Go's GMP scheduler model. What are M, P, and G and how do they interact? What is work stealing in Go's scheduler and why does it matter for performance? Why can Go run millions of goroutines while equivalent OS-thread workloads fail? What is the difference between unbuffered and buffered channels in Go? What happens when you send to, receive from, or close a nil or closed channel? How does the select statement work in Go and what are its key properties? What is a data race in Go, how do you detect it, and what are the three main fixes? When do you use sync.Mutex versus sync.RWMutex, and what are the critical usage rules? How does sync.WaitGroup work and what are the most common mistakes? What is sync.Once and what guarantees does it provide? What causes deadlocks in Go and how do you detect and prevent them? What are directional channels in Go and why use them in function signatures? What causes goroutine leaks and how do you prevent and detect them? Implement fan-out and fan-in concurrency patterns in Go. How do you use a nil channel in select to dynamically enable or disable cases? When should you use the sync/atomic package instead of sync.Mutex? Implement a bounded worker pool pattern in Go. What are the differences between time.After, time.NewTimer, and time.NewTicker, and which leaks resources? How does context.Context enable clean goroutine cancellation and why is 'defer cancel()' critical? How do you use a buffered channel as a semaphore to limit goroutine concurrency? What is the 'done channel' pattern and why has context.Context largely superseded it? What is the goroutine loop-variable capture bug and how do you fix it? How does golang.org/x/sync/errgroup simplify concurrent error handling? When should a channel carry 'chan struct{}' versus a typed value, and why is close() used for broadcast? What is GOMAXPROCS, how does it affect parallelism, and what is the container pitfall? How does Go's select handle multiple ready cases, and how do you implement true priority? What is sync.Cond and when do you use it instead of channels? Implement Go's canonical pipeline pattern with cancellation from the Go blog. What is Go's memory model and why does it matter for concurrent code? What is the check-then-act race condition (TOCTOU) and how do you fix it? What is asynchronous preemption in Go (1.14+) and why was it introduced? How do you safely use a map from multiple goroutines in Go? How do you use a buffered channel as a task queue with natural backpressure? How does Go handle goroutines that make blocking syscalls — what happens to M and P? Implement a simple publish-subscribe broker using Go channels. What is the lock-held-during-I/O anti-pattern and how do you fix it? Write a complete example of implementing operation timeouts in Go using select. How do you write tests that detect goroutine leaks automatically? Implement a concurrent word count across multiple files — a classic Go interview puzzle. How does GOMAXPROCS=1 change behaviour and when is it actually useful? How do you implement a high-performance sharded concurrent map in Go? What are the specific happens-before guarantees for channel operations in Go's memory model? How do you implement a hedged request pattern using select and goroutines? How does sync.Pool reduce GC pressure in high-throughput Go services? What is a livelock and how does it differ from a deadlock in Go programs? How do you implement backpressure in Go to prevent overloading downstream systems? Implement a lock-free stack using atomic CAS operations and explain the ABA problem. Summarise: channel vs mutex decision guide, and the top concurrency pitfalls.
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