Barrier

Definition

A barrier is a synchronization point where each participating thread waits until all participating threads have arrived.

Core idea

Barriers are useful when a parallel algorithm proceeds in phases.

Typical pattern:

1. each thread computes its part of phase 1
2. all threads wait at the barrier
3. only after everyone has arrived may any thread start phase 2

This is needed when later work depends on the fact that the whole group has finished the previous step.

Example intuition

Suppose multiple threads update different rows of a matrix.

• in phase 1, each thread computes new values for its assigned rows
• before phase 2 starts, all rows must be finished
• the barrier ensures no thread starts reading phase-2 data too early

Without a barrier, one thread could continue with incomplete data produced by other threads.

Barrier vs other synchronization mechanisms

• a lock protects a critical section so only one thread accesses shared state at a time
• notify is used for condition-based signaling between threads
• a barrier coordinates a whole group of threads at the same synchronization point

So a barrier is about collective progress, not mutual exclusion.

Java example with CyclicBarrier

import java.util.concurrent.*;
 
int n = 4;
CyclicBarrier barrier = new CyclicBarrier(n);
 
Runnable worker = () -> {
    try {
        doPhase1Work();
        barrier.await();   // wait until all n workers arrive
        doPhase2Work();
    } catch (InterruptedException | BrokenBarrierException e) {
        throw new RuntimeException(e);
    }
};

barrier.await() blocks until all registered parties have called it.
Once the last thread arrives, they are released and can continue.

Common Java tools

• CyclicBarrier: reusable barrier for a fixed number of threads
• Phaser: more flexible barrier with multiple phases and dynamic registration

Failure mode

If one participating thread never reaches the barrier, the others may wait forever.

Important

A barrier only works correctly when all participating threads agree on the same phases and all of them eventually reach the barrier.