(Reprint) visual explanation of processes and threads

process （process） And thread （thread） Is the basic concept of the operating system , But they're more abstract , It's not easy to master .

1.

The core of a computer is CPU, It does all the computing . It's like a factory , Running at all times . A factory may have multiple workshops to complete different tasks , For example, manufacturing workshop , Assembly workshop, etc , These different workshops correspond to many computer applications , This can be seen in the task manager .

2.

And we play games while listening to music on weekdays, which is really the use of multi processes , But actually CPU Just switch between different processes frequently , Let's feel CPU Running both programs at the same time . So back to the factory example , When there is only one workshop working in the whole factory , It is a single process .

3.

Suppose the power of the factory is limited , It can only be used in one workshop at a time . in other words , When a workshop starts , All other workshops have to be shut down . The meaning behind it is , Single CPU You can only run one task at a time , In the real computer ,CPU That's exactly what it did , The process is like the workshop of a factory , It represents CPU The single task that can be handled . Any moment ,CPU Always run a process , Other processes are not running .

In a workshop , There can be a lot of workers . They worked together on a task .

5.

Threads are like workers in a workshop . A workshop can include multiple workers , Of course, a workshop can have only one worker , For example, quality inspectors or maintenance personnel in highly automated production workshops , But a workshop cannot be without a worker on duty , Just like a process cannot exist without a thread , That is, we often say that a process can have multiple threads , A process must have at least one thread , That is, the main thread .

6.

The workshop space is shared by the workers , For example, many rooms are accessible to every worker . This symbolizes that the memory space of a process is shared , Each thread can use these shared memory , For example, pile .

7.

But , The size of each room is different , Some rooms can only hold one person at most , Such as toilet . When there are people inside , No one else can go in . This means that when a thread uses some shared memory , Other threads have to wait for it to finish , To use this memory , For example, stack .

8.

A simple way to prevent others from entering , Just add a lock to the door , This is very similar to the toilet on the train . First come lock the door , The next person sees the lock , Just line up at the door , Don't go in until the lock is open . This is called " The mutex "（Mutual exclusion, abbreviation Mutex）, Prevent multiple threads from reading and writing a block of memory area at the same time .

9.

And some rooms , Can accommodate at the same time n personal , For example, kitchen. , Or more vividly, you go out to eat , A packed , You can only wait in line for a vacant seat . in other words , If the number of people is greater than n, The extra people can only wait outside . It's like some memory area , Only a fixed number of threads can be used .

10.

Solution at this time , Just hang it at the door n Key to . Those who go in take a key , Hang the key back when you get out . Those who arrived later found that the key was on the air , I knew I had to wait in line at the door . This is called " Semaphore "（Semaphore）, To ensure that multiple threads do not conflict with each other .

It's not hard to see. ,mutex yes semaphore A special case of （n=1 when ）. in other words , We can replace the former with the latter . however , because mutex It's simpler , And high efficiency. , So when we have to guarantee the exclusive use of resources , Or this design .

11.

The design of the operating system , So it comes down to three points ：

（1） In the form of multi process , Allow multiple tasks to run at the same time , You watch movies and hang up playing games at the same time ;

（2） In the form of multithreading , Allow a single task to run in different parts , You can type and chat with others while sending files through social software ;

（3） Provide coordination mechanisms , On the one hand, it prevents conflicts between processes and threads , On the other hand, it allows resources to be shared between processes and threads .

（ End ） Most of them are reprinted , A few added their own understanding , It's standing on the shoulders of giants , If the description is not in place , Please point out that .
Link to the original text ： A simple explanation of process and thread