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Golang / Golang Internals and Memory Management Interview Questions

What is the difference between sync.Mutex and sync.RWMutex and when do you use each?

sync.Mutex is a mutual-exclusion lock: at most one goroutine holds the lock at any time, whether reading or writing. sync.RWMutex distinguishes readers from writers: multiple readers can hold the lock simultaneously (RLock), but a writer requires exclusive access (Lock).

// sync.Mutex — use when all accesses are writes or complex read+write ops
type SafeCounter struct {
    mu sync.Mutex
    count int
}
func (c *SafeCounter) Inc() {
    c.mu.Lock()
    defer c.mu.Unlock()
    c.count++
}

// sync.RWMutex — use when reads dominate
type Config struct {
    mu   sync.RWMutex
    data map[string]string
}
func (c *Config) Get(key string) string {
    c.mu.RLock()          // many readers can hold RLock concurrently
    defer c.mu.RUnlock()
    return c.data[key]
}
func (c *Config) Set(key, val string) {
    c.mu.Lock()           // exclusive — no readers or writers
    defer c.mu.Unlock()
    c.data[key] = val
}

// Important: RWMutex is NOT always faster than Mutex
// RWMutex has higher per-operation overhead than Mutex
// It wins when: read:write ratio >> 10:1 and lock is held for a notable duration
// For very short critical sections, Mutex can be faster

// TryLock (Go 1.18+)
if c.mu.TryLock() {
    defer c.mu.Unlock()
    // got the lock
} else {
    // lock is held — do something else
}

Writer starvation: in Go's RWMutex, when a writer requests the lock, new readers are blocked even if current readers still hold RLock. This prevents writers from being starved by a continuous stream of readers. The writer waits only for the existing readers to finish, then gets exclusive access.

When is sync.RWMutex more efficient than sync.Mutex?
In Go's RWMutex, what happens to new read-lock (RLock) requests when a writer is waiting?

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