并发 vs. 并行
Concurrency means multiple tasks which start, run, and complete in overlapping time periods, in no specific order. Parallelism is when multiple tasks OR several part of a unique task literally run at the same time, e.g. on a multi-core processor. Remember that Concurrency and parallelism are NOT the same thing.
并发是指多个任务，它们在重叠的时间段内（没有特定的顺序）开始，运行和完成。 并行是指多个任务或唯一任务的多个部分同时运行的情况，例如，在多核处理器上。 请记住，并发和并行性不是一回事。
Let’s understand more in detail that what I mean when I say Concurrency vs. Parallelism.
Concurrency is essentially applicable when we talk about minimum two tasks or more. When an application is capable of executing two tasks virtually at same time, we call it concurrent application. Though here tasks run looks like simultaneously, but essentially they MAY not. They take advantage of CPU time-slicing feature of operating system where each task run part of its task and then go to waiting state. When first task is in waiting state, CPU is assigned to second task to complete it’s part of task.
当我们讨论最少两个或更多任务时，并发基本上是适用的。当一个应用程序能够同时执行两个任务时，我们称之为并发应用程序。虽然这里的任务看起来是同时运行的，但本质上它们可能不是。它们利用了操作系统的CPU 时间切片 特性，每个任务运行其部分任务，然后进入等待状态。当第一个任务处于等待状态时，CPU被分配给第二个任务来完成任务的一部分。
Operating system based on priority of tasks, thus, assigns CPU and other computing resources e.g. memory; turn by turn to all tasks and give them chance to complete. To end user, it seems that all tasks are running in parallel. This is called concurrency.
Parallelism does not require two tasks to exist. It literally physically run parts of tasks OR multiple tasks, at the same time using multi-core infrastructure of CPU, by assigning one core to each task or sub-task.
Parallelism requires hardware with multiple processing units, essentially. In single core CPU, you may get concurrency but NOT parallelism.
Now let’s list down remarkable differences between concurrency and parallelism.
Concurrency is when two tasks can start, run, and complete in overlapping time periods. Parallelism is when tasks literally run at the same time, eg. on a multi-core processor.
Concurrency is the composition of independently executing processes, while parallelism is the simultaneous execution of (possibly related) computations.
Concurrency is about dealing with lots of things at once. Parallelism is about doing lots of things at once.
An application can be concurrent – but not parallel, which means that it processes more than one task at the same time, but no two tasks are executing at same time instant.
An application can be parallel – but not concurrent, which means that it processes multiple sub-tasks of a task in multi-core CPU at same time.
An application can be neither parallel – nor concurrent, which means that it processes all tasks one at a time, sequentially.
An application can be both parallel – and concurrent, which means that it processes multiple tasks concurrently in multi-core CPU at same time .
That’s all about Concurrency vs. Parallelism, a very important concept in java multi-threading concepts.
Happy Learning !!