Synchronization Patterns • Barrier SynchronizationMedium⏱️ ~2 min
What is a Barrier?
Definition
A barrier is a synchronization point where all threads must arrive before any can proceed. The first N-1 threads wait; the Nth thread releases everyone.
The Marathon Start Line
Imagine runners at a marathon. Everyone gathers at the start line. No one can begin until all runners are ready. The last runner to arrive triggers the starting gun, and everyone runs together. That start line is a barrier.
Barrier vs Rendezvous
Rendezvous is a barrier for exactly 2 threads. A barrier generalizes this to N threads. Same concept, bigger scale. Every thread does its first phase of work, hits the barrier, waits for everyone, then proceeds to the second phase.
Barrier: All Must Arrive Before Any Proceed
Common Use Cases
Parallel algorithms: All threads compute partial results, barrier, then combine results. Simulations: All agents move one step, barrier, then check for collisions. Testing: Ensure all threads reach a point before checking invariants.
Key Property: No thread executes code after the barrier until ALL threads have reached the barrier. This is a strong synchronization guarantee.
💡 Key Takeaways
✓A barrier makes all N threads wait until everyone arrives. First N-1 block, the Nth releases all.
✓Barrier generalizes rendezvous from 2 threads to N threads. Same pattern, larger scale.
✓Common in parallel algorithms: compute phase, barrier, combine phase. Ensures all partial results ready.
✓Barriers guarantee no thread proceeds past the sync point until all have reached it.
✓Used in simulations, parallel computing, testing, and any scenario requiring phased execution.
📌 Examples
1Matrix multiplication: Each thread computes rows, barrier, then threads can read each others results for next phase.
2Game physics: All objects move, barrier, then collision detection runs. Cannot check collisions mid-movement.
3Map-reduce: All mappers finish, barrier, then reducers start. Reducers need complete mapper output.