When should you use the sync/atomic package instead of sync.Mutex?

The sync/atomic package provides lock-free operations on individual primitive values using CPU-level instructions (LOCK prefix on x86, load-linked/store-conditional on ARM). It is faster than a mutex for simple single-variable operations but is limited to supported types and single-variable updates.

import "sync/atomic"

// AtomicInt64 — no lock needed
var counter int64
atomic.AddInt64(&counter, 1)           // atomic increment
v := atomic.LoadInt64(&counter)         // atomic read
atomic.StoreInt64(&counter, 0)          // atomic write

// Compare-and-Swap (CAS) — the foundation of lock-free algorithms
swapped := atomic.CompareAndSwapInt64(&counter, 0, 100)
// Sets counter=100 ONLY if counter==0; returns true if swap happened
fmt.Println(swapped, atomic.LoadInt64(&counter)) // true 100

// atomic.Value — atomically store/load any interface value
var config atomic.Value
config.Store(&Config{Timeout: 30 * time.Second}) // must always store same concrete type
cfg := config.Load().(*Config)

// Performance comparison (approximate, uncontended):
// atomic.AddInt64:      ~5  ns
// sync.Mutex Lock+Unlock: ~25 ns (uncontended)
// sync.Mutex Lock+Unlock: ~200 ns (contended)

// Use atomic when:
// ✓ Simple counters, flags, sequence numbers
// ✓ Publishing a single pointer or value others read
// ✓ High-frequency metrics (LongAdder / sharded counter pattern)

// Use Mutex when:
// ✓ Multiple related variables must be updated together atomically
// ✓ Non-trivial read-modify-write patterns
// ✓ Protecting complex types like maps or slices

Cache-line false sharing: when multiple atomic variables are stored in adjacent memory, a write to one invalidates the CPU cache line of another — degrading performance even though different variables are being modified. Pad hot atomic variables to separate cache lines (64 bytes on x86) in high-performance code.

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