Though the availability of concurrency in the hardware is most obvious with multiprocessor or multicore systems, some processors can execute multiple threads on a single core. The important factor to consider is the number of hardware threads, which is the measure of how many independent tasks the hardware can genuinely run concurrently. Even with a system that has genuine hardware concurrency, it's easy to have more tasks than the hardware can run in parallel, so task switching is still used in these cases. For example, on a typical desktop computer there may be hundreds of tasks running, performing background operations, even when the computer is nominally idle. It's the task switching that allows these background tasks to run and you to run your word processor, compiler, editor, and web browser (or any combination of applications) all at once. Figure 1.2 shows task switching among four tasks on a dual-core machine, again for an idealized scenario with the tasks divided neatly into equally sized chunks. In practice, many issues will make the divisions uneven and the scheduling irregular. Some of these issues are covered in chapter 8 when we look at factors affecting the performance of concurrent code.

Figure 1.2. Task switching of four tasks on two cores

All the techniques, functions, and classes covered in this book can be used whether your application is running on a machine with one single-core processor or with many multicore processors, and are not affected by whether the concurrency is achieved through task switching or by genuine hardware concurrency. But as you may imagine, how you make use of concurrency in your application may well depend on the amount of hardware concurrency available. This is covered in chapter 8, where I discuss the issues involved in designing concurrent code in C++.

1.1.2. Approaches to concurrency

Imagine, for a moment, a pair of programmers working together on a software project. If your developers are in separate offices, they can go about their work peacefully, without being disturbed by each other, and they each have their own set of reference manuals. But communication isn't straightforward; rather than turning around and talking to each other, they have to use the phone or email, or get up and walk to the other's office. Also, you have the overhead of two offices to manage and multiple copies of reference manuals to purchase.