Why multithreading programming?

A lightweight process (lightweight)
Process, lwp) is the smallest unit of the program execution flow. A process is the basis of the operating system structure. It is an ongoing program. an instance of a program running in a computer can be allocated to an entity that is executed by a processor; the execution in a single order shows the activity units described by the current status and a group of related system resources.

Thread technology was proposed as early as 1960s, but the real application of multithreading to the operating system was in the middle of 1980s, and Solaris was a leader in this field. Traditional UNIX also supports the concept of thread, but only one thread is allowed in a process, so multithreading means multi-process. Now, multithreading technology has been supported by many operating systems, including Windows/NT and, of course, Linux.

Why do we need to introduce threads after the concept of a process is introduced? What are the advantages of multithreading? What systems should adopt multithreading? We must answer these questions first.

One of the reasons for using multithreading is that compared with processes, it is a very "frugal" multi-task operation method. We know that in a Linux system, starting a new process must be allocated to it with an independent address space, creating a large number of data tables to maintain its code segment, stack segment, and data segment, this is an "expensive" way of multitasking. While multiple threads running in a process use the same address space for each other to share most of the data. The space required to start a thread is much less than the space required to start a process, in addition, the time required for switching between threads is much less than the time required for switching between processes. According to statistics, in general, the overhead of a process is about 30 times the overhead of a thread. Of course, this data may be quite different in a specific system.

The second reason for using multithreading is the convenient communication mechanism between threads. For different processes, they have independent data space, and data transmission can only be performed through communication. This method is not only time-consuming, but also inconvenient. The thread is not the case. Because the threads in the same process share data space, the data of one thread can be directly used by other threads, which is fast and convenient. Of course, data sharing also brings about other problems. Some Variables cannot be modified by two threads at the same time, and some subprograms declare static data, which is more likely to cause catastrophic damage to multithreaded programs, these are the things you need to pay attention to when writing multi-threaded programs.

In addition to the advantages mentioned above, multi-threaded programs, as a multi-task and concurrent working method, certainly have the following advantages:

1) Improve application response. This is especially meaningful for graphic interface programs. When an operation takes a long time, the entire system will wait for this operation. At this time, the program will not respond to keyboard, mouse, and menu operations, but will use multithreading technology, putting time consuming in a new thread can avoid this embarrassing situation.

2) Make the multi-CPU system more effective. The operating system ensures that different threads run on different CPUs when the number of threads is not greater than the number of CPUs.

3) Improve the program structure. A long and complex process can be considered to be divided into multiple threads and become several independent or semi-independent running parts. Such a program will facilitate understanding and modification.