What are the relationships and differences between threads and processes

Thread definition

Thread is the basic execution unit of a process , All tasks of a process are executed in threads

The process wants to perform the task , There must be threads , The process must have at least one thread

The program will start a thread by default , This thread is called the main thread or UI Threads

Process definition

A process is an application running in the system

Each process is independent , Each process runs in its own dedicated and protected memory

The difference between a process and a thread

address space ： Threads of the same process share the address space of this process , And there is an independent address space between processes .

Resource ownership ： Threads in the same process share the resources of the process （ Such as memory 、I/O、cpu etc. ）, But the resources between processes are independent .

After a process crashes , No impact on other processes in protected mode , But when a thread crashes, the whole process dies . So multiprocessing is more robust than multithreading .

Process switching , It consumes a lot of resources , Efficient . So when it comes to frequent switching , It's better to use threads than processes . Also, if concurrent operations of some variables are required to be performed at the same time and shared at the same time , Only threads, not processes

Execution process ： Each independent process has an entry for the program to run 、 Sequential execution sequence and program entry . But the thread cannot execute independently , Must exist in the application , Multiple thread execution control provided by the application .

Threads are the basic unit of processor scheduling , But the process is not .

advantage

It can improve the efficiency of program execution
It can improve the utilization rate of resources （CPU, Memory ）
When the task on the thread is finished , The thread will automatically destroy

shortcoming

Starting a thread requires a certain amount of memory space （ By default , Every thread takes up 512 KB）
If you start a lot of threads , It takes up a lot of memory space , Reduce the performance of the program
More threads ,CPU The higher the overhead on the calling thread
Programming is more complicated , For example, communication between threads 、 Multithreaded data sharing