Parallelism and concurrency

We all know , In the dimension of operating system , Thread is CPU The smallest unit of allocation .

Parallelism is the same time , Both threads are executing . This requires two CPU To execute two threads separately . It can be understood as two CPU Service two threads , Threads and servers are one-to-one .

Concurrency is the same time , There's only one execution , But for a period of time , Both threads execute . The implementation of concurrency depends on CPU Switching thread , Because the switching time is very short , Therefore, it is basically imperceptible to users . You can think of it as one CPU Serving two threads ,CPU Switch back and forth between two threads .

The essence of concurrency is a physics CPU（ It can also be multiple Physics CPU） Multiplexing between several programs , Concurrency is to enforce multi-user sharing of limited physical resources to improve efficiency .

Parallelism refers to two or more events or activities occurring at the same time , In a multiprogramming environment , Parallelism enables multiple programs to work at different times at the same time CPU At the same time .

For example, go to the canteen to cook , Parallel is to learn from multiple students and eat at the same time in multiple windows , Concurrency means that multiple students are eating at the same time in the same window .

Parallelism requires two or more threads running on different processors , Concurrency can be switched on a processor through time slices .