The key term to concurrency is the non-deterministic order of execution of multiple threads. Non-deterministic means that you run the program one time, the ord

The key term to concurrency is the non-deterministic order of execution of multiple threads. Non-deterministic means that you run the program one time, the order of execution is: thread 3, thread 5, thread 1, thread 4, and thread 2. You run it another time, the order can be: thread 1, thread 5, thread 2, thread 4, and thread 3. In another time with 2 CPU: thread 1/5, thread 1/3 (1 overlaps 5 and 3), thread 2, and thread 4. From your perspective, the OS schedules your threads "randomly." It might be due to system load, interrupt, or timing. Doesn't matter. It's just that the order is not guaranteed. The goal of writing a proper concurrent program is no matter what the order of execution, the computed outcome is the same, due to consistency guarantee coded in.