问题
What is the differene between concurrency and multithreading? Is concurrency only possible in multicore cpu? can anybody explain it with an example?
回答1:
What is the differene between concurrency and multithreading?
Concurrency describes the way in which processes run. They are either sequential (one after another), concurrent (able to make progress "at the same time" although not necessarily at the same instant), or parallel (they happen simultaneously).
Multi-threading is a technique which allocates individual threads of execution; they are essentially lightweight processes with some advantages with respect to shared resources from their parent.
If you pay close attention, multi-threading is possible on both concurrent and non-concurrent systems. A thread is a lightweight process (with respect to processes); so, having multiples of threads on a non-concurrent system would not result in parallel programming. They would still start and run until finished before the other. And on a concurrent system they would each get their fair share at some CPU time; they would all be making progress concurrently.
Is concurrency only possible in multicore cpu?
I think we know now, the answer to this is no. Concurrent execution of processes is taken for granted to the point it's widely misunderstood as parallelism; a much more powerful tool.
To give an example that provides some insight, think about your machine. It does all kinds of stuff all the time and you do not (hopefully) experience any lag in its performance. All these processes are running concurrently giving you, the user, a perception of parallelism even when on a single core machine (I know cause I'm old :)).
But what about a merge sort? Couldn't we perform two merge sorts simultaneously on two halves of the data; yes. But only if we have multiple cores/CPUs.
回答2:
Concurrency means doing multiple tasks simultaneously. It means multiple tasks are running parallely. So definitely to run multiple tasks parallely you need multiple threads.
So Concurrency is achieved by Multithreading
Now coming to your Question :
Is concurrency only possible in multicore cpu?
The answer is No.
If I have 2 threads and only 1 core. In this case, CPU will give time to each thread to complete its task. So Multithreading is even possible in single core CPU.
来源：https://stackoverflow.com/questions/35100102/what-is-the-differene-between-concurrency-and-multithreading