The difference between parallelism and concurrency

parallel

parallel （Parallel）, When the system has more than one CPU when , When one CPU When executing a process , the other one CPU Another process can be executed , The two processes do not preempt each other CPU resources , You can do it at the same time , This way we call parallel (Parallel).

In fact, the determinant of parallelism is not CPU The number of , It is CPU The number of cores , For example, a CPU Multiple cores can also be parallel .

Concurrent

Concurrent （Concurrent）, In the operating system , It means that several programs in a period of time are between the start and the completion of running , And these programs are all running on the same processor .

Concurrency is not really “ At the same time ”, It's just CPU Divide a time period into several time segments （ Time interval ）, And then switch back and forth between these time intervals , because CPU The processing speed is very fast , As long as the time interval is handled properly , It makes users feel that multiple applications are running at the same time . Such as ： Playing games and listening to music are both completed from the beginning to the end on the same computer in the same time period . that , It can be said that listening to music and playing games are concurrent .

The difference between concurrency and parallelism

• Concurrent , It refers to multiple things , It happened at the same time . parallel , It refers to multiple things , At the same time .

• Multiple concurrent tasks preempt each other's resources . Parallel tasks do not preempt each other's resources .

• Concurrency is that multiple programs run simultaneously in a period of time , Parallelism is at some point , There are really multiple programs running .

• Only in many CPU Or a CPU In the case of multicore , It's going to happen in parallel . otherwise , What seems to happen at the same time , In fact, they are all concurrent .