Race conditions, threads, processes, tasks, queues in iOS

What is a race condition?

When you have two threads changing a variable simultaneously. It’s possible to get unexpected results. Imagine a bank account where one thread is subtracting a value to the total and the other is adding a value.

The order of events goes like this:

1. Access the total to subtract to it. (total = 500, expense = 40)
2. Compute the difference. (total =500, newTotal = 460)
3. Save the new total. (total = 460)
4. Access the total to add to it. (total = 460, income = 200)
5. Compute the sum (total = 460, newTotal = 660)
6. Save the new total (total = 660)

Great! 660 is exactly what we expected, but it could have gone like this:

1. Access the total to subtract to it. (total = 500, expense = 40)
2. Access the total to add to it. (total = 500, income = 200)
3. Compute the difference. (total =500, newTotal = 460)
4. Save the new total. (total = 460)
5. Compute the sum (total = 500, newTotal = 700)
6. Save the new total. (total = 700)

So the account is now at 700. That’s not what we expected. Uh oh.

When should we suspect a race condition?

When variables that have multiple threads operating on them and you’re getting unexpected results some of the time.

How do we fix this?

One option is to lock the variable total so that only one thread could operate on it at a time, the lock is called a mutex.

But in iOS, you want to use grand central dispatch (GCD) or OperationQueues to handle your tasks. So in the example of the bank total, we’d probably just want to use a serial dispatch queue which has sole access to the total variable. By using GCD or OperationQueues, you’re letting Apple do the thread management and removing the code for locking. Less code is good.

So what are threads, processes and tasks?

The task is an abstract concept for describing something you want to do. In the example above, adding money to the total or subtracting money from the total are both tasks. They use a process to actually affect the change in code.

A process keeps track of what needs to be done and delegates the tasks to the threads. A process can have one or multiple threads. The process is like a project manager and the thread is like a worker.

What’s the difference between hardware and software threads?

The threads we’ve been talking about so far have been software threads. They’re (generally) independent units of computation.

The hardware threads are based on the number of cores on the computer. For example, the current 12 inch MacBook has this processor with a 1.1GHz two-core processor. That means it have 2 cores, each with 1.1GHz of clock speed. Each of those core have two 2 hardware threads, so there are 4 hardware threads total.

Each of these hardware threads can run many software threads, depending on how the operating system uses them.