Process communication and thread explanation

2.6 Process of communication

• Process of communication ： Refers to the exchange of information between processes
• Basic communication ： Only some states and a small amount of data are exchanged between processes . Such as ： Mutual exclusion and synchronization between processes

Disadvantages of semaphore mechanism as a communication tool ：

• 1、 Low efficiency
• 2、 Communication is not transparent to users

Advanced communications ： A large amount of data can be exchanged between processes

• Users can directly use a set of communication commands provided by the operating system , A group of communication methods that efficiently transmit large amounts of data .
• The operating system hides the details of process communication , Transparent to users , It reduces the complexity of communication programming .

Process of communication （ Advanced communications ）

• One 、 The type of process communication
• Two 、 The implementation of message passing communication
• 3、 ... and 、 Some problems in the implementation of message passing system
• Four 、 Message buffer queue communication mechanism

One 、 The type of process communication

Advanced communication mechanisms can be divided into three categories ：

• Shared memory system
• Messaging system
• Pipeline communication system

1、 Shared memory system

Processes that communicate with each other share certain data structures or shared storage areas , Communicate through these spaces .

• Communication mode based on shared data structure （ Basic communication ）
  Processes share certain data structures , So as to realize the information exchange between processes
  Realization ：
  Setting of public data structure and processing of inter process synchronization , It's all the responsibility of programmers .
  operating system – Provide shared memory
  characteristic ： Inefficient . It is only suitable for transmitting a relatively small amount of data .

• Communication mode based on shared memory area
  Mark out a shared storage area in the storage area , Processes can communicate by reading or writing data in shared memory .

2、 Messaging system

• Data exchange between processes , In formatted messages .
• Programmers directly use a set of communication commands provided by the system （ The original language ） communicate .
• example ： computer network ： Network message

3、 Pipeline communication

Two 、 The implementation of message passing communication

1、 Using direct communication primitives to solve producer problems - Consumer issues

When a producer produces a commodity （ news ） after , Easy to use send The primitive sends the message to the consumer process ; Consumer processes take advantage of receive The primitive gets a message . If the news hasn't come out yet , Consumers have to wait , Until the producer process sends the message .

2、 Indirect communication ------ Communicate through mailbox

• The mailbox is used to temporarily store messages sent by the sending process to the target process , The receiving process takes the message sent to itself from the mailbox .
• Messages can be safely saved in the mailbox , Only approved target users are allowed to read at any time
• Use mailbox , It can communicate in real time , And non real-time communication .
• Mailboxes can be created by the operating system , It can also be created by user processes , The creator is the owner of the mailbox .

Mailbox classification ：

   Private mailbox    User processes create their own    A one-way 
   Public mailbox    Operating system creation    two-way 
   Shared mailbox    A process is created 

The difference between public mailbox and shared mailbox ： The creator is different .

3、 ... and 、 Some problems in the implementation of message passing system

1、 communication link

classification ：

（1） According to the establishment mode of communication link ：

• Explicit connection ： First use “ Establishing a connection ” command （ The original language ） Establish a communication link ; communicate ; Then remove the link in an explicit way . For computer networks .
• Implicit connection ： The sending process does not need to explicitly put forward the requirements for establishing links , Directly use the sending command provided by the system （ The original language ）, The system will automatically establish a link for it . For stand-alone systems .

2、 Format of message

• Stand alone system environment ： The environment is the same , The message format is simple
• Computer network environment ： The environment is different , The transmission distance of the message is very long, and the message format is relatively complex .

3、 Process synchronization mode

When communicating between processes , Supplemented by process synchronization mechanism , Enable coordinated communication between processes .

The sending process or receiving process completes the sending or receiving of the message , There are two possibilities ： Process or continue sending （ receive ） Or blocked .

    The sending process is blocked 、 The receiving process is blocked    （ Tight synchronization   There is no buffer , The data sent by the sending process is directly received by the receiving process ）
    The sending process is not blocked 、 The receiving process is blocked   （ Sending speed < Reception speed ）
    Neither the sending process nor the receiving process is blocked      （ The sending speed is similar to the receiving speed , Basic synchronization ）

Four 、 Message buffer queue communication mechanism

principle ：

（2）PCB Data item about communication in

2、 Send primitive

3、 Receive the original

2.7 Basic concept of thread

Thread Introduction

1、 Two basic properties of a process

（1） A process is an independent unit that can have resources
（2） A process is a basic unit that can be independently scheduled and dispatched

This makes the process a basic unit that can run independently , It forms the basis of process concurrent execution .

2、 The time and space cost of program concurrent execution

• Process owns resources , Make the creation of the process 、 Revocation and scheduling cost a lot of time and space .
• This limits the number of processes set up in the system , And process switching should not be too frequent , This limits the further improvement of concurrency .
• therefore , Separate the two attributes of the process from the system , among The second property is determined by the thread To achieve .

3、 Threads —— As the basic unit of scheduling and allocation

1. Light entity
2. The basic unit of independent scheduling and dispatching
3. Can be executed concurrently
4. Share process resources

Comparison between thread and process

1. The basic unit of dispatch
   When introducing threads OS in , Thread has been regarded as the basic unit of scheduling and dispatching , Therefore, thread is the basic unit that can run independently . When threads switch , Only a small amount of register contents need to be saved and set , The switching cost is much lower than the process . In the same process , Thread switching will not cause process switching , But when switching from a thread in one process to a thread in another process , It will inevitably cause process switching .
2. concurrency
   When introducing threads OS in , Not only can processes execute concurrently , Moreover, multiple threads in a process can execute concurrently , It even allows all threads in a process to execute concurrently . Again , Threads in different processes can also execute concurrently .
3. Has the resources
   Processes can have resources , And as a basic unit of resources in the system . However, threads do not own system resources , But only a little essential 、 Resources that can guarantee independent operation .
4. independence
   The independence between different threads in the same process is much lower than that between different processes . Because it is necessary to prevent mutual interference and destruction between processes , Each process has an independent address space and other resources , In addition to sharing global variables , Do not allow other processes to access . But different threads in the same process are often created to improve concurrency and cooperate with each other , They share the memory address space and resources of the process , For example, each thread can access all addresses in the process address space to which they belong , For example, the stack of a thread can be read and written by other threads , Even completely clear . Files opened by one thread can be read by other threads 、 Write .
5. overhead
   The cost of thread creation and revocation is much lower than that of the process , During thread switching , The cost is also far lower than the process .
6. Support multiprocessor systems

About the basic state of threads , be ready 、 Execution and blocking are similar to processes

About thread control block TCB, And PCB similar .

• Thread identifier , Give each thread a unique thread identifier
• A set of registers , Including program registers PC, Contents of status register and general register
• Thread running state , Used to describe the running state of a thread
• priority , Describes the priority of thread execution
• Thread specific storage , Used to store on-site protection information during thread switching , And statistics related to the thread
• Signal shielding , Shield some signals
• Stack pointer , When the thread is running , Procedure calls are often made

Thread control block TCB, Used to record all information used to control and manage threads .

Multithreading OS The process of has the following properties

• A process is a basic unit of owning resources
• Multiple threads can execute concurrently
• The process is no longer an executable entity