Spinlocks vs. Mutexes: When to Spin and When to Sleep
3 days ago
- #spinlocks
- #synchronization
- #mutexes
- Spinlocks and mutexes are synchronization primitives with different trade-offs: spinlocks burn CPU while waiting, mutexes sleep and involve syscalls.
- Spinlocks are efficient for very short critical sections (under 100ns) with low contention, as they avoid the overhead of context switches.
- Mutexes are better for longer critical sections (over 10μs) or high contention, as they allow threads to sleep and free up CPU resources.
- Spinlocks can cause priority inversion and cache line bouncing, leading to inefficiencies in preemptible contexts.
- Mutexes in glibc use a fast path with atomic operations before resorting to syscalls, making uncontended mutexes very fast (~25-50ns).
- Real-world systems like Redis use spinlocks for nanosecond operations, while PostgreSQL uses mutexes for longer operations.
- Performance can be monitored using tools like perf, strace, and /proc filesystem to measure context switches, cache misses, and syscalls.
- The choice between spinlocks and mutexes depends on critical section duration, contention level, and real-time requirements.