Multithreading (I) processes and threads

 

Catalog

  One 、 process

1、 What is a process

2、 How the operating system manages processes

Two 、 Threads

1、 What is thread

2、 Why have threads

  3、 ... and 、 Similarities and differences between process and thread

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One 、 process

1、 What is a process

         A process is an abstraction of the operating system for a running program , In other words , You can think of a process as a running process of a program .

         In the file , We can often see some suffixes as .exe file , These documents are called " Executable file ". Once such documents are executed , The operating system will load the file into memory ,CPU Will execute some instructions inside this file , At this point, these running applications are processes . 

2、 How the operating system manages processes

（1） Describe a process

         Specify some related attributes of a process , Mainly through the use of the operating system c/c++ Of " Structure " To describe ( In the operating system, it is called process control block PCB).

（2） Several processes

        Typical implementation ： Use a two-way linked list to put the PCB String together , It is convenient to add, delete, check and modify

among ," Create a process " Is to create PCB, Then take it. PCB Add to the two-way linked list ;" Destruction process " Is to find the... On the linked list PCB, Delete from the linked list ;" View Task Manager " Is to traverse the linked list .

（3） Interprocess communication

         Processes are independent of each other , But it can also interact through some means . The operating system provides many " public space "( Generally file operation and network operation ), process A Put data in public space , The process then B Then take it away , This is interprocess communication .

（4）PCB Some properties in  

        ① pid： It's the process id, Process identification id.

        ② Memory pointer ： Indicates the code to be executed by the process / Where are the instructions in memory , And where the data that the process depends on (.exe The operating system will put .exe Load into memory , Become a process ).

        ③ Document descriptor table ： As long as every file opened by the process , An item will be added to the file descriptor table . The file descriptor table can be treated as an array , Each of these elements is a structure . A process just starts , The system will automatically open the standard input , standard output , Standard error three files .

        ④ state ： Describes how the current process should be scheduled next .

        Ready state : Can be executed at any time .

         Blocked state : You can't go to... For the time being CPU On the implementation .

        ⑤ priority ： When we have many tasks , Which task to allocate time to first , Which task will you assign time to ; Which task has more time , Which task has less time , That's the priority .

        ⑥ Billing information ： Count how long each process has been executed 、 What instructions were executed 、 How long did you wait in line . Provide guidance for process scheduling .

        ⑦ Context ： Indicates that the last time the process was scheduled CPU The state of , In the next process CPU You can return to your previous state when you , Then continue to execute . Similar to archiving + Reading .

The process is called out CPU Before , First of all CPU The data in all registers in is saved to memory , Equivalent to archiving

The next time the process is scheduled CPU When , It can be recovered from memory to register , Equivalent to reading files

 （5） Process scheduling

         Process scheduling : The process scheduler is the real core of the operating system , It is directly responsible for CPU The distribution of . All processes in the system are in CPU Running on , Process scheduling is their switch

Two 、 Threads

1、 What is thread

         A thread is an internal part of a process , The process contains threads . If you think of the process as a factory , So thread is the production line inside the factory . A factory can have a production line , There can also be multiple production lines .

2、 Why have threads

        Because the system supports multitasking , So we need to do " Concurrent programming ".

         Although concurrent programming can also be achieved through multiple processes , But frequent creation / The cost of the destruction process is very high , And the cost of frequent scheduling process is also very high （ Because the creation process requires the application of resources , The destruction process requires the release of resources , This in itself is very inefficient ）

        resolvent ： Using threads to implement concurrent programming . Threads are lighter , Each thread performs a task , You can also program concurrently . establish 、 The destruction 、 The cost of scheduling threads is lower than that of processes ( The process needs to apply for the release of resources , Threads are included in the process , Share the same resource with other threads , Re create threads without reallocating resources . But it's not that the more threads, the better , When there are few tasks , Multithreading will reduce efficiency )

  3、 ... and 、 Similarities and differences between process and thread

1、 Processes contain threads . At least one thread per process exists , The main thread .

2、 There is no shared memory space between processes . Threads of the same process share the same memory space .

3、 Operating system creation process , To allocate resources to a process , Process is the basic unit of resource allocation in the operating system ;

Thread creation operating system , Is to be in CPU Scheduled execution on , Thread is the basic unit of operating system scheduling thread execution .

4、 The process is independent . Each process has its own virtual address space , There is a problem with a process , No impact on other processes . Multiple threads in the same process use the same memory space , If a thread has a problem , It is likely to affect other threads , It may also directly lead to problems in the whole process .