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Thinking in C++ Vol 2 - Practical Programming
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One of the most compelling reasons for using concurrency is to produce a responsive user interface. Consider a program that performs some CPU-intensive operation and thus ends up ignoring user input and being unresponsive. The program needs to continue performing its operations, and at the same time it needs to return control to the user interface so that the program can respond to the user. If you have a quit button, you don t want to be forced to poll it in every piece of code you write in your program. (This would couple your quit button across the program and be a maintenance headache.) Yet you want the quit button to be responsive, as if you were checking it regularly.

A conventional function cannot continue performing its operations and at the same time return control to the rest of the program. In fact, this sounds like an impossibility, as if the CPU must be in two places at once, but this is precisely the illusion that concurrency provides (in the case of multiprocessor systems, this may be more than an illusion).

You can also use concurrency to optimize throughput. For example, you might be able to do important work while you re stuck waiting for input to arrive on an I/O port. Without threading, the only reasonable solution is to poll the I/O port, which is awkward and can be difficult.

If you have a multiprocessor machine, multiple threads can be distributed across multiple processors, which can dramatically improve throughput. This is often the case with powerful multiprocessor web servers, which can distribute large numbers of user requests across CPUs in a program that allocates one thread per request.

A program that uses threads on a single-CPU machine is still just doing one thing at a time, so it must be theoretically possible to write the same program without using any threads. However, multithreading provides an important organizational benefit: The design of your program can be greatly simplified. Some types of problems, such as simulation a video game, for example are difficult to solve without support for concurrency.

The threading model is a programming convenience to simplify juggling several operations at the same time within a single program: The CPU will pop around and give each thread some of its time.[148] Each thread has the consciousness of constantly having the CPU to itself, but the CPU s time is actually sliced among all the threads. The exception is a program that is running on multiple CPU. But one of the great things about threading is that you are abstracted away from this layer, so your code does not need to know whether it is running on a single CPU or many.[149] Thus, using threads is a way to create transparently scalable programs if a program is running too slowly, you can easily speed it up by adding CPUs to your computer. Multitasking and multithreading tend to be the most reasonable ways to utilize multiprocessor systems.

Threading can reduce computing efficiency somewhat, but the net improvement in program design, resource balancing, and user convenience is often quite valuable. In general, threads enable you to create a more loosely coupled design; otherwise, parts of your code would be forced to pay explicit attention to tasks that would normally be handled by threads.

Thinking in C++ Vol 2 - Practical Programming
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   Reproduced courtesy of Bruce Eckel, MindView, Inc. Design by Interspire