Everyone waits for everyone
A barrier synchronizes a group of threads at a common point. Each thread that reaches the barrier blocks until the whole group has arrived, then all are released together.
How it counts
- The barrier knows the expected party size.
- Each arriving thread increments an arrival count and waits.
- When the count equals the party size, the barrier wakes every waiter.
This is ideal for phased computation. In a simulation, all threads must finish updating phase one before any reads neighbors in phase two. The barrier guarantees no thread races ahead.
The hazard
If a thread crashes or never arrives, the others wait forever. Barriers therefore assume a fixed, known set of participants that all reliably reach the meeting point.
Key idea
A barrier makes every thread pause until the full group arrives, then releases them together, which keeps phased parallel work in lockstep but stalls forever if a participant goes missing.