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Golang / GoLang Concurrency Mastery Interview Questions

Why can Go run millions of goroutines while equivalent OS-thread workloads fail?

Three fundamental differences make goroutines dramatically cheaper than OS threads: initial stack size, scheduling cost, and blocking behaviour.

Goroutine vs OS Thread
AspectOS ThreadGoroutine
Initial stack1–8 MB (fixed, kernel-allocated)2 KB (grows dynamically up to 1 GB)
SchedulingPreemptive by OS (expensive context switch with full register save)Cooperative + async-preemptive by Go runtime (user-space, cheap)
BlockingBlocks the entire OS thread on syscallParks goroutine; OS thread freed for other goroutines
Creation cost~10 µs, requires kernel call~300 ns, entirely user-space
Practical limit~10,000 before memory exhaustion~1,000,000+ on standard hardware
package main

import (
    "fmt"
    "runtime"
    "sync"
)

func main() {
    const n = 1_000_000
    var wg sync.WaitGroup
    wg.Add(n)
    for i := 0; i < n; i++ {
        go func() {
            defer wg.Done()
            // Each goroutine: ~2 KB stack at creation
            // 1M goroutines ≈ 2 GB total — feasible on modern hardware
        }()
    }
    wg.Wait()
    fmt.Println("done, goroutines:", runtime.NumGoroutine())
}

The small initial stack is possible because Go uses copy-on-grow stacks: when a goroutine needs more stack space the runtime allocates a new, larger stack, copies the old contents, updates all internal pointers, and frees the old stack. This is transparent to user code.

What is the default initial stack size for a newly spawned goroutine in Go?
Which function returns the current number of live goroutines in a running Go program?

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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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