“同步和异步是编程模型。并发和并行是任务执行的方式……” 来源:https://medium.com/better - programming/sync vs -异步并发- vs - vs - 5754 cdb60f66平行

换句话说，同步和异步描述了在进行函数调用时程序如何执行(它是等待还是继续执行?)，而并发和并行描述了函数(任务)将如何执行(并发=可能同时执行，并行=有效地同时执行)。

Concurrency Concurrency means that an application is making progress on more than one task at the same time (concurrently). Well, if the computer only has one CPU the application may not make progress on more than one task at exactly the same time, but more than one task is being processed at a time inside the application. It does not completely finish one task before it begins the next. Parallelism Parallelism means that an application splits its tasks up into smaller subtasks which can be processed in parallel, for instance on multiple CPUs at the exact same time. Concurrency vs. Parallelism In Detail As you can see, concurrency is related to how an application handles multiple tasks it works on. An application may process one task at at time (sequentially) or work on multiple tasks at the same time (concurrently). Parallelism on the other hand, is related to how an application handles each individual task. An application may process the task serially from start to end, or split the task up into subtasks which can be completed in parallel. As you can see, an application can be concurrent, but not parallel. This means that it processes more than one task at the same time, but the tasks are not broken down into subtasks. An application can also be parallel but not concurrent. This means that the application only works on one task at a time, and this task is broken down into subtasks which can be processed in parallel. Additionally, an application can be neither concurrent nor parallel. This means that it works on only one task at a time, and the task is never broken down into subtasks for parallel execution. Finally, an application can also be both concurrent and parallel, in that it both works on multiple tasks at the same time, and also breaks each task down into subtasks for parallel execution. However, some of the benefits of concurrency and parallelism may be lost in this scenario, as the CPUs in the computer are already kept reasonably busy with either concurrency or parallelism alone. Combining it may lead to only a small performance gain or even performance loss. Make sure you analyze and measure before you adopt a concurrent parallel model blindly.

从http://tutorials.jenkov.com/java-concurrency/concurrency-vs-parallelism.html

当一个经理有几个员工，并且可以给他们每个人一个单独的任务时，并行性就会发生。工人们做他们的工作，并向经理提供结果。如果任务不能完全分离，例如任务之间的结果有一定的依赖性，或者需要在没有其他推理的情况下使用相同的资源，那么并行度就受到这些约束，不能完全实现。

Concurrency happens when a manager has several tasks but only less workers, hence some workers are given more than one task. Any worker given multiple tasks, divides each original given task to several steps and does the steps interleaved, each task result will be given back to manager as soon as every steps of it finished. Manager receive a task result while other tasks started and progressed several steps but have not finished yet. If any worker with multiple task decides not to start a single step of a given task before finishing every steps of an already started task, this is called sequentiality.

Asynchrony is any of the two above mixed or separated, seen from the manager's point of view. When the manager assigns the tasks to either few or enough workers he shall not be awaited stalled until any results are given back. He can do his personal jobs or whatever, while jobs are progressing. Usually workers do not decide how tasks should be divided into steps. An inversion of control means manager decides over steps and gives single steps to workers. So when he receives a step result from a worker, give him another step, maybe from another task. The whom under control is responsible for composing back step results into task results as well. So Asynchronicity comes with responsibility for control and probably coordination. If any worker is urged to work sequentially, from manager's point of view he is a synchronous worker.

Summary As it's simple to guess, full parallelism is an unrealisable idea unless otherwise in rare mostly trivial cases. Since reality comes with interdependent tasks and shared resources and lack of workers. So concurrency is the reality. From manager's point of view this concurrency is best if it does not hinder him from fine controlling the tasks, and if positive it is called asynchronous. Also computer software engineering best practices, augmented by S in SOLID principle, historically made servers single step runners called micro-services, this returned back control to the clients. So current situation is concurrency from server point of view and asynchronicity from client point of view.

并发性意味着同时执行多个任务，但不一定是同时执行。当你必须执行多个任务，但你只有一个资源时，我们就会追求并发性。在单核环境中，并发是通过上下文切换实现的。

并行性就像同时执行多个任务，就像你可以一边唱歌一边洗澡。现在你在并行地做这些任务。

术语异步与线程执行有关。在异步模型中，当执行一个任务时，您可以切换到另一个任务，而无需等待前一个任务完成。

异步编程帮助我们实现并发性。多线程环境中的异步编程是实现并行的一种方法。

总结

并发(Concurrent):多件事情似乎同时发生(具有在并发任务之间快速切换的能力;它们是并发的，因为每个任务都想要一块资源，CPU等等。)

并行是指同时发生多件事情(执行线程的数量与执行内核的数量密切相关)

异步是简单的非阻塞，当我们必须等待的事情不会让我们忙于等待(需要某种通知机制从我们离开的地方继续)

并发是指多个任务的执行是交错的，而不是每个任务依次执行。

并行性是指这些任务实际上是并行执行的。

异步是一个单独的概念(尽管在某些上下文中是相关的)。它指的是一个事件可能发生在与另一个事件不同的时间(不同步)。下面的图表说明了同步执行和异步执行之间的区别，其中参与者可以对应于不同的线程、进程甚至服务器。