Grand Central Dispatch vs NSThreads?

Question

I searched a variety of sources but don\'t really understand the difference between using NSThreads and GCD. I\'m completely new to the OS X platform so I might

Answer 1

While the answers so far are about the context of threads vs GCD inside the domain of a single application and the differences it has for programming, the reason you should always prefer GCD is because of multitasking environments (since you are on MacOSX and not iOS). Threads are ok if your application is running alone on your machine. Say, you have a video edition program and want to apply some effect to the video. The render is going to take 10 minutes on a machine with eight cores. Fine.

Now, while the video app is churning in the background, you open an image edition program and play with some high resolution image, decide to apply some special image filter and your image application being clever detects you have eight cores and starts eight threads to process the image. Nice isn't it? Except that's terrible for performance. The image edition app doesn't know anything about the video app (and vice versa) and therefore both will request their respectively optimum number of threads. And there will be pain and blood while the cores try to switch from one thread to another, because to avoid starvation the CPU will eventually let all threads run, even though in this situation it would be more optimal to run only 4 threads for the video app and 4 threads for the image app.

For a more detailed reference, take a look at http://deusty.blogspot.com/2010/11/introducing-gcd-based-cocoahttpserver.html where you can see a benchmark of an HTTP server using GCD versus thread, and see how it scales. Once you understand the problem threads have for multicore machines in multi-app environments, you will always want to use GCD, simply because threads are not always optimal, while GCD potentially can be since the OS can scale thread usage per app depending on load.

Please, remember we won't have more GHz in our machines any time soon. From now on we will only have more cores, so it's your duty to use the best tool for this environment, and that is GCD.