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 be completely misinterpreting this.

From what I read online, GCD seems to do the exact same thing as basic threads (POSIX, NSThreads etc.) while adding much more technical jargon (“blocks”). It seems to just overcomplicate the basic thread creation system (create thread, run function).

What exactly is GCD and why would it ever be preferred over traditional threading? When should traditional threads be used rather than GCD? And finally is there a reason for GCD’s strange syntax? (“blocks” instead of simply calling functions).

I am on Mac OS X 10.6.8 Snow Leopard and I am not programming for iOS – I am programming for Macs. I am using Xcode 3.6.8 in Cocoa, creating a GUI application.

Answer 1

Advantages of Dispatch

The advantages of dispatch are mostly outlined here:

Migrating Away from Threads

The idea is that you eliminate work on your part, since the paradigm fits MOST code more easily.

• It reduces the memory penalty your application pays for storing thread stacks in the application’s memory space.
• It eliminates the code needed to create and configure your threads.
• It eliminates the code needed to manage and schedule work on threads.
• It simplifies the code you have to write.

Empirically, using GCD-type locking instead of @synchronized is about 80% faster or more, though micro-benchmarks may be deceiving. Read more here, though I think the advice to go async with writes does not apply in many cases, and it’s slower (but it’s asynchronous).

Advantages of Threads

Why would you continue to use Threads? From the same document:

It is important to remember that queues are not a panacea for
replacing threads. The asynchronous programming model offered by
queues is appropriate in situations where latency is not an issue.
Even though queues offer ways to configure the execution priority of
tasks in the queue, higher execution priorities do not guarantee the
execution of tasks at specific times. Therefore, threads are still a
more appropriate choice in cases where you need minimal latency, such
as in audio and video playback.

Another place where I haven’t personally found an ideal solution using queues is daemon processes that need to be constantly rescheduled. Not that you cannot reschedule them, but looping within a NSThread method is simpler (I think). Edit: Now I’m convinced that even in this context, GCD-style locking would be faster, and you could also do a loop within a GCD-dispatched operation.

Blocks in Objective-C?

Blocks are really horrible in Objective-C due to the awful syntax (though Xcode can sometimes help with autocompletion, at least). If you look at blocks in Ruby (or any other language, pretty much) you’ll see how simple and elegant they are for dispatching operations. I’d say that you’ll get used to the Objective-C syntax, but I really think that you’ll get used to copying from your examples a lot 🙂

You might find my examples from here to be helpful, or just distracting. Not sure.