1. The difference between program and process

Let's start with a story ：

A Usually because of busy work , They eat in the company canteen or take out food . But one day, the class suddenly wanted to cook by themselves , therefore A Go shopping in the supermarket , He bought some ingredients to make a scrambled egg with tomatoes . When you get home ,A Took out a book that was placed in the deepest part of the bookshelf 《 Recipes for home cooked dishes 》, Follow the above method , He began to prepare the dishes ： First wash the tomatoes and cut them into pieces , Stir the eggs into egg liquid , Boiling hot oil ........ Finally in the A Under the scornful efforts of , A tomato scrambled egg with all the colors, flavors and flavors is finished .

In the above story , We can abstract out a simple computing processing model ：

【 Program 】 That's the recipe （ Stored on the shelf , A kind of Physics There is , Describe the process and method of cooking in a specific form ）
notes ：
Famous computer scientist Niklaus Wirth Put forward ： Program = data structure + Algorithm
Think of recipes as programs , It is in the form of pictures and texts （ data structure ） Vividly show the readers how to cook each dish （ Algorithm ）
【 process 】 This is a series of activities for cooking （ dynamic On going ）, Including washing vegetables 、 Chopping vegetables 、 Stir fry and so on
【CPU】 Namely A I am , He is the executor of all operations
【 data 】 It's all kinds of raw materials for scrambled eggs with tomatoes , Including tomatoes 、 egg ...（ Pass a series of data through CPU Processing becomes different data ）

2. process

2.1 State of process

A process is a sequence of programs on a data set Activities .
The process has three states , Namely ：

Running state 【running】（actually using the CPU at that instant）
The ready state 【ready】（runnable; temporarily stopped to let another process run）
Blocking state 【blocked】（unable to run until some external event happens）

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From the above , We can clearly see the transformation process between the three states ：

Happen when IO In operation , The current process will be blocked until IO Operation is completed
When the scheduler interrupts the current process and executes another process , The original process will be ready
When the scheduler selects the process , It will change from ready state to running state , enjoy CPU resources
When IO After completion , The blocked process becomes ready , Waiting for the operating system to schedule

2.2 Process context switch

When a process interrupts from one process to another （interrupt） Or system call （system call） Context switching occurs
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Process in progress P0 Because the terminal or system call will cpu The right to use is returned to os
os preservation P0 Of PCB Information , And reload from memory P1 Process pcb Content , And implement P1
P1 After execution , Given to by an interrupt or system call os（ Scheduler scheduler / Dispatcher scheduler）
Come back and execute P0（ load P0 Of PCB Information ）

The cost of context switching is actually very high , Requires a lot of processor time （ So we usually use assembly complete ）, occasionally 1 Context switching can be performed thousands of times in seconds 【CONTEXT SWITCH】

Even modern computers are reducing the amount of context switching CPU Time , But that is only in CPU Clock cycles decrease , Faster processing , Instead of improving the efficiency of context switching

3. Why introduce the concept of threads ？

Now that we have a process , So why did computer scientists come up with the concept of a thread ？

In the system, multiple applications will be carried out at the same time , They often switch states , By splitting a process into multiple threads , achieve Quasi parallel 【quasi-parallel】, The program model will be simpler （ These threads Share address space and data resources , These capabilities are exactly what multiple processes cannot do （ Address space is different ））
More lightweight . Compared to process , It's easier （ Fast ） Create and destroy （ In some systems , Faster than process creation 10-100 times ）
Better performance . When there are a lot of calculations and I/O When dealing with , Threads have the ability to overlap these activities , Speed up the program
More useful for multi-core processors .

【 notes 】
because process Is the owner of resources , So in creating 、 revoke 、 The switching operation requires large space-time overhead , Limits the further improvement of concurrency .

by Reduce the overhead of process switching , Treat the process as Resource allocation unit and Dispatching unit These two properties Separate the , That is, the process is the basic unit of resource allocation , But not as the basic unit of scheduling （ Rarely scheduled or switched ）, Assign the responsibility of scheduling execution and switching to “ Threads ”.

The benefits of doing so can not only Improve the concurrency of the system , It can also adapt to the new symmetric multiprocessor （SMP） The operation of the environment , Give full play to its performance

3.1 Some defects of threads

Although the thread runs very fast , But it is not perfect
Mainly in the Security Problems in ： We know , The threads in the process share data and address space , If An error occurred in one of the threads （ For example, a variable is rewritten by mistake ）, Other threads will also have problems , Errors that lead to the final whole process

therefore , We say that the current use of processes and threads is also separate , Not blindly pursuing threads ：

stay High performance computing （ Like the weather forecast 、 Water conservancy 、 Aerodynamics, etc ）, This category needs to pursue high-performance computing , At the same time, the program is not error prone , Just fit Using threads
And for example browser This application , Because the operation of the browser page is performed by the user , Sometimes data security is considered , So recommend Use process , A process opens a web page

3.2 Implementation of threads

3.2.1 User space implementation

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Create in user space Thread library , Provides a set of procedures for managing threads by

Runtime system 【run-time system】 To complete thread management

The kernel manages processes , The kernel is unaware of the existence of threads

Thread switching There's no need to get into the kernel

Such as Unix、Linux

advantage	shortcoming
Fast switching speed	A process has only one thread running on the processor
The scheduling algorithm can be set by the application	If a thread of the a process calls a blocked system call , Then all threads of the process will also be blocked , Page invalidation can have the same problem
User level threads can run on any operating system , Including unsupported threaded operating systems

3.2.2 Kernel space implementation

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The kernel manages all thread management , establish , Revocation and dispatch **, And provide... To the application API

The kernel maintains the process and thread context

Thread switching Need kernel support

Thread based To schedule

advantage	shortcoming
Scheduled by the kernel , When there are multiple processors , Multiple threads of a process can execute simultaneously on multiple processors	Created by the kernel , revoke , Dispatch , More overhead
Blocking of one thread of a process does not cause blocking of other threads , The same goes for page invalidation

3.3.3 Hybrid implementation

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Threads establish stay —— User space

Threads Dispatch stay —— Kernel space

4. The relationship between processes and threads that have resources ？

【 process 】 yes Resource allocation The unit of ——————【 Threads 】 yes CPU Dispatch The unit of

Threads Also known as “ Lightweight process ”【lightweight processes】

it （ All threads of the same process ） Shared process address space , At the same time, there are some unique information ：

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The process information shared by each thread is shown on the left side of the above figure

but , Every Threads also have their own PC、 register 、 Stack 、 State information

（ Because each thread may call different steps ,

therefore Each thread has its own stack 、PC、 register , This prevents exceptions from occurring during execution ）

【 notes 】

Multiple threads within a process coexist
Share information with each other , It is even possible for one thread to create or delete another thread
Although there is a hierarchical relationship between processes , But there are no threads （ All threads are Equal ）