Program Thread 和 Process的不同点

Thread is for execution
- Kernel level thread, physical parallelism
  - Cores Divide work amount of physical cores / CPU
  - Load balancing
  - Data Splitting
    - Which will lead to data dependency coodination and message passing
  - Bad thing for multiple cores is this is hard for Testing and debugging
- User level thread only have logical parallelism
  - Example, Read from user input is blocking; we have to work on it to make it logical parallelism
Process is for resource

What is Program

Program是一个存在disk中且断电或重启不会消失可执行文件，存储在存储媒介中，以实体文件的形态存在

A program is an executable file residing on the disk (secondary storage) in a directory. It is also termed as a set of instructions stored in the secondary storage device that are intended to carry out a specific job. It is read into the primary memory and executed by the kernel.

本文来自 Abbymz 的CSDN 博客，全文地址请点击：https://blog.csdn.net/Abby210/article/details/51225034?utm_source=copy

或者可以称之为persistently 的文件，断电也不会消失。

What is Process ?

当program被执行之后，就变成了进程。执行者的权限和程序所需的资料都会载入到内存中，OS会给予这些内存单元一个pid。

Unit of resource ownership (allocation) and unit of protection

text， Code of Programmer
Data
Heap
Stack

一个program可以创建出很多个进程，关机或停电会死掉

Process的组成

Permission user or kernel
Priority
Files
ID
- PID
- PPID
- Folk
- UserID
Protected access to
- Processors
- other process
- Files
- I/O
A virtual space that holds the process image

What is Thread ?

线程称之为Lightweight Process，一个进程可以有多个线程，他们共享一片内存

has access to the same data
- When one thread alters a data, other threads see the results
- When one thread open a file, other threads can also access that file.

Unit of dispatch or unit of execution

多线程的好处

开启线程要快于开启进程
关闭线程要快于关闭进程
切换线程要快于切换进程
线程可以相互通讯

什么叫Multithreading

The ability of an OS to support multiple, concurrent paths of execution within a single process.

Process and thread states

Ready
- new process usually set as read state
- At this time scheduler is not pick it up yet.
  - If it get picked by scheduler and running in CPU, then goes to Running State
  - If it get external event, something like lock, then goes to block
Running,
- Scheduler picked it up, and CPU is run this instruction
  - If it get finished, goes to Exit states
  - If it get IO interrupts, goes to ready, blocked status
  - If it can not get the resource which is conflict with other process, goes to deadlock
Block
- If it lock (wait able ) is release, or IO is completed, then goes to Ready
Deadlock
Exit (Zombie State)
- When process is finished, waiting for cleaning up

How Deadlock happen ?

When process A has resource a, and it need resource b from process B.
At the same time, process B need resource a from A
Then those process gonna have deadlock due to both of them can get the resource they need.