Multithreading via GCD on iOS/Mac

Question

I'm looking to multithread some code in Swift, but I'm wondering about how GCD handles threading. Does each new thread I create automatically run on a separate core (if available), or does it use another mechanism for assigning threads to cores?

Essentially what I want is to prevent the new threads I create from running concurrently on the same core; I want to guarantee they are on different cores (again, in the case they available).

I'm not writing an app for the App Store or anything, I just need it to run these threads on different cores on my multicore machine. From what I could find online, GCD decides for itself whether to run threads on different cores, but I could not gather whether this was simply based on availability or something else entirely. Thanks.

Answer 1

If you use either the Grand-Central Dispatch API (dispatch_async, etc.) or higher-level APIs that use GCD (like NSOperationQueue), GCD will pick the best thread for you, and therefore the best core for you, depending on a variety of factors. You only have control over the GCD queue, not the thread or the processor core.

You can also create your own threads using NSThread, but this generally isn't recommended unless you need to run your code on pre-GCD operating systems. I think the only way to guarantee two different cores are used is to have some code running on mainThread and another on a detached thread. As far as I know, the even-more-granular control you're requesting isn't available via public APIs. (I'm reasonably sure it's possible using private APIs, but I don't know how.)