Processes and threads - concepts and process scheduling

Program 、 The concept of process and thread

Program （program）： Instructions + Data composition , It usually exists in the form of documents

process （process）：

• A process is an abstract result of the execution of a program , Each program supports the existence of multiple processes at the same time （ It can be seen as a recipe and a cooking process ）
• （ The fundamental difference between a process and a thread ） Processes are operating system resources （ Hardware resources ） Assigned base unit

Threads （thread）：

• Each process is composed of one or more threads
• （ The fundamental difference between a process and a thread ） A thread is a processor （CPU） The basic unit of task scheduling and execution

Coordination and management of core hardware

CPU ： Allocate in time —— Process scheduling （ Process management ）

Memory ： Allocate in units of space —— memory management

Process management （ Process scheduling ）

1. process ： From the perspective of computer hardware, it is a set of related data （ An object ）

class PCB ProcessControlBlock {
    //1. Unique identifier     pid
    //2. Some information associated with the program （ For example, the program file path ）
    //3. Some data in the operation stage （ user 、 Start path, etc ）
    //4. Statistics of allocated resources 
    //5. About scheduling related 
}

OS Internal management List<PCB> perhaps Map<pid, PCB> To achieve the purpose of managing the process

2. Process scheduling

（1） State of process

The ready state ： Everything is ready for the process , Just assign it CPU

Running state ： The process has been assigned to CPU 了 （ It's already running ）

（ General OS The implementation , There is no breakdown of these two states ）

The third category is divided into the following categories ：

• Initial state ： You just created , Not ready yet
• Ending state ： All instructions have been executed ,PCB The object has not been OS Recycling
• Blocking state ： There are some problems during operation （ wait for IO ）

（2） State transition diagram

Be careful ：

1. Why? Running state Only through The ready state arrive ？

• Not once it's ready , There is CPU Distribute
• Only in ready state , Can be assigned CPU

2. What conditions may lead to changes from the operating state to other states ？

• end of execution
• The time slice runs out
• Preempted by a high priority process
• Wait for external events （IO）

（3） Site protection 、 Field recovery and selection algorithm

What is on-site protection 、 Scene recovery ？

1. The scene ：CPU Important data in registers in （ The core part is PC）
2. Protect ： Save the corresponding data in memory （PCB There is a corresponding location in the object to save ）
3. recovery ： Restore the previously protected data in memory to CPU In the register

CPU visual angle （ Only instructions , But I don't know whose instruction it is ）

According to which algorithm to select a process for allocation

The scheduling algorithm used in batch processing system ：

Measures ： throughput , Turnaround time ,CPU utilization , Fair balance .

1. First come first serve algorithm
2. Short job preferred
3. Highest response ratio first algorithm

The scheduling algorithm adopted by the interactive system ：

Measures ： response time , Fair balance .

1. Time slice rotation scheduling algorithm
2. Virtual rotation method
3. Priority scheduling algorithm
4. Multilevel feedback queue scheduling algorithm

（4）OS In context Concurrent （concurrent）VS parallel （parallel）

Concurrency is logically simultaneous （simultaneous）, And parallel is physical simultaneity .

Concurrent （ False simultaneity ）： Refers to a processor that processes multiple tasks at the same time .

From a human perspective , process 1、2、3 stay “ meanwhile ” Conduct

parallel （ True simultaneous ）： It refers to multiple processors or multi-core processors processing multiple different tasks at the same time .

 “ real ” There are multiple CPU , Running processes at the same time 1、2、3

 （5） User mode （user space） VS Kernel mode （kernel space）

  The difference between open permissions ： Kernel mode The permission of is relatively large , User mode Limited authority

analogy ： Go to ATM To withdraw money from

1. Card insertion
2. Input password
3. Verify that the password is correct                                         Inside
4. Verify if there is a balance + Permission withdrawal                             nucleus
5. Spit money into the till                                                     state
6. Spit money
7. Tuka
8. Take the money and the card

（5） Process of communication

Common form ： The Conduit 、 Shared memory 、 The Internet 、 Semaphore 、 The signal

（6） Multi process model VS Multithreading model （ Multithreading under the same process ）

remarks ： Standing on the OS Context , and Java The context is not completely consistent

• Processes are isolated , Threads are not isolated
• Communication between multiple processes is troublesome , Multithreaded communication is simple
• A process crash often causes the same process to stop ,OS Processing errors are generally handled in process units （Java The thread of has improved this ）
• The multi process model is more stable , But communication is troublesome , It will take up slightly more resources ; The multithreaded model is less stable , Lighter