Evaluating Spin Locks

 

Evaluating Spin Locks

To understand how effective a spin lock is, we evaluate it using three important criteria:

  1. Correctness

  2. Fairness

  3. Performance

These criteria help us decide when spin locks are useful and when they are not.

1. Correctness (Mutual Exclusion)

The most important requirement of any lock is correctness.

Question:

Does the spin lock ensure mutual exclusion?

Answer:

✅ Yes

A spin lock allows only one thread to enter the critical section at a time.

This is guaranteed because the lock is acquired using an atomic instruction
(such as test-and-set or compare-and-swap).

Conclusion:

✔ Spin locks are correct and safely protect critical sections.

2. Fairness (Starvation)

Fairness refers to whether every waiting thread will eventually get the lock.

Question:

Is a waiting thread guaranteed to enter the critical section?

Answer:

❌ No

Spin locks do not provide fairness guarantees.

  • A thread may spin indefinitely

  • Under heavy contention, some threads may starve

  • There is no ordering (like FIFO) for lock acquisition

Result:

⚠ Spin locks are unfair and can cause starvation

3. Performance

Performance depends heavily on the system configuration.

Case 1: Single-CPU System ❌ (Poor Performance)

Imagine:

  • One thread holds the lock

  • That thread is preempted while inside the critical section

  • Other threads try to acquire the lock

What happens?

  • Other threads spin continuously

  • Each spinning thread wastes an entire time slice

  • CPU cycles are wasted doing no useful work

Result:

❌ Spin locks perform very poorly on single-core systems

Case 2: Multi-CPU System ✅ (Good Performance)

Now imagine:

  • Thread A runs on CPU 1

  • Thread B runs on CPU 2

  • Thread A holds the lock briefly

  • Thread B spins on another CPU

What happens?

  • Thread B spins only for a short time

  • Lock is released quickly

  • Minimal CPU wastage

Result:

✔ Spin locks perform well on multicore systems
✔ Especially effective when:

  • Critical sections are short

  • Number of threads ≈ number of CPUs

4. Summary Table

Evaluation AspectSpin Lock Behavior
Correctness    ✅ Ensures mutual exclusion
Fairness    ❌ No fairness, starvation possible
Performance (Single CPU)    ❌ Very poor
Performance (Multi CPU)    ✅ Good
Waiting Style    Busy waiting (spinning)

5. When Should Spin Locks Be Used?

✅ Use Spin Locks When:

  • Running on multicore systems

  • Critical section is very short

  • Context switching cost is high

❌ Avoid Spin Locks When:

  • Single-core systems

  • Long critical sections

  • High lock contention

6. Key Takeaway 

Spin locks are simple and correct but unfair. They waste CPU cycles on single-core systems but work efficiently on multicore systems when critical sections are short.

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