concurrency parallel 并发 并行

Computer Systems A Programmer's Perspective Second Edition

The general phenomenon of multiple flows executing concurrently is known

as

concurrency

. The notion of a process taking turns with other processes is also

known as

multitasking

. Each time period that a process executes a portion of its

flow is called a

time slice

. Thus, multitasking is also referred to as

time slicing

.For

example, in Figure 8.12, the flow for Process A consists of two time slices.

 

Notice that the idea of concurrent flows is independent of the number of

processor cores or computers that the flows are running on. If two flows overlap

in time, then they are concurrent, even if they are running on the same processor.

However, we will sometimes find it useful to identify a proper subset of concurrent

flows known as

parallel flows

. If two flows are running concurrently on different

processor cores or computers, then we say that they are

parallel flows

, that they

are

running in parallel

, and have

parallel execution

.