Golang / GoLang Concurrency Mastery Interview Questions

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

// Use Mutex when:
// â Multiple related variables must be updated together atomically
// â Non-trivial read-modify-write patterns
// â 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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What does atomic.CompareAndSwapInt64(&v, old, new) do?It atomically swaps the values stored at two int64 pointers

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It sets *v = new only if *v currently equals old; returns true if the swap happened — all as one atomic operation

✓ Correct! Well done.

It atomically computes new = old + *v and stores the result

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It compares old and new and stores the larger value

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Why prefer sync.Mutex over atomic when updating multiple related struct fields?Mutex is always faster than atomic operations

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Atomic operations update one variable at a time — updating multiple related fields requires a mutex so the entire update is seen as one consistent operation

✓ Correct! Well done.

atomic.Value cannot store structs with more than 4 fields

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Mutex operations are automatically optimised by the Go compiler

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Golang / GoLang Concurrency Mastery Interview Questions

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

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