How to articulate the difference between asynchron

Question

Many platforms promote asynchrony and parallelism as means for improving responsiveness. I understand the difference generally, but often find it difficult to articulate in my own mind, as well as for others.

I am a workaday programmer and use async & callbacks fairly often. Parallelism feels exotic.

But I feel like they are easily conflated, especially at the language design level. Would love a clear description of how they relate (or don't), and the classes of programs where each is best applied.

Answer 1

Asynchronous Let's say you are the point of contact for your client and you need to be responsive i.e. you need to share status, complexity of operation, resources required etc whenever asked. Now you have a time-consuming operation to be done and hence cannot take this up as you need to be responsive to the client 24/7. Hence, you delegate the time-consuming operation to someone else so that you can be responsive. This is asynchronous.

Parallel programming Let's say you have a task to read, say, 100 lines from a text file, and reading one line takes 1 second. Hence, you'll require 100 seconds to read the text file. Now you're worried that the client must wait for 100 seconds for the operation to finish. Hence you create 9 more clones and make each of them read 10 lines from the text file. Now the time taken is only 10 seconds to read 100 lines. Hence you have better performance.

To sum up, asynchronous coding is done to achieve responsiveness and parallel programming is done for performance.

Answer 2

This article explains it very well: http://urda.cc/blog/2010/10/04/asynchronous-versus-parallel-programming

It has this about asynchronous programming:

Asynchronous calls are used to prevent “blocking” within an application. [Such a] call will spin-off in an already existing thread (such as an I/O thread) and do its task when it can.

this about parallel programming:

In parallel programming you still break up work or tasks, but the key differences is that you spin up new threads for each chunk of work

and this in summary:

asynchronous calls will use threads already in use by the system and parallel programming requires the developer to break the work up, spinup, and teardown threads needed.

Answer 3

I tend to think of the difference in these terms:

Asynchronous: Go away and do this task, when you're finished come back and tell me and bring the results. I'll be getting on with other things in the mean time.

Parallel: I want you to do this task. If it makes it easier, get some folks in to help. This is urgent though, so I'll wait here until you come back with the results. I can do nothing else until you come back.

Of course an asynchronous task might make use of parallelism, but the differentiation - to my mind at least - is whether you get on with other things while the operation is being carried out or if you stop everything completely until the results are in.

Answer 4

My basic understanding is:

Asynchonous programming solves the problem of waiting around for an expensive operation to complete before you can do anything else. If you can get other stuff done while you're waiting for the operation to complete then that's a good thing. Example: keeping a UI running while you go and retrieve more data from a web service.

Parallel programming is related but is more concerned with breaking a large task into smaller chunks that can be computed at the same time. The results of the smaller chunks can then be combined to produce the overall result. Example: ray-tracing where the colour of individual pixels is essentially independent.

It's probably more complicated than that, but I think that's the basic distinction.

Answer 5

Generally, there are only two ways you can do more than one thing each time. One is asynchronous, the other is parallel.

From the high level, like the popular server NGINX and famous Python library Tornado, they both fully utilize asynchronous paradigm which is Single thread server could simultaneously serve thousands of clients (some IOloop and callback). Using ECF(exception control follow) which could implement the asynchronous programming paradigm. so asynchronous sometimes doesn't really do thing simultaneous, but some io bound work, asynchronous could really promotes the performance.

The parallel paradigm always refers multi-threading, and multiprocessing. This can fully utilize multi-core processors, do things really simultaneously.

Answer 6

When you run something asynchronously it means it is non-blocking, you execute it without waiting for it to complete and carry on with other things. Parallelism means to run multiple things at the same time, in parallel. Parallelism works well when you can separate tasks into independent pieces of work.

Take for example rendering frames of a 3D animation. To render the animation takes a long time so if you were to launch that render from within your animation editing software you would make sure it was running asynchronously so it didn't lock up your UI and you could continue doing other things. Now, each frame of that animation can also be considered as an individual task. If we have multiple CPUs/Cores or multiple machines available, we can render multiple frames in parallel to speed up the overall workload.