What's the difference between the "global queue" and the "main queue" in GCD?

Question

Among some other ways, there are these two ways to get queues in GCD:

dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);  dispatch_get_main_queue(); 

If I'm not completely wrong, the "main queue" is executing on the main thread and is good for "callback" blocks which execute UI work.

Does this mean a "global queue" is one that runs on a background thread?

Answer 1

The main queue does indeed run on the main thread like you say.

The global queues are concurrent queues and from the main page for dispatch_get_global_queue:

Unlike the main queue or queues allocated with dispatch_queue_create(), the global concurrent queues schedule blocks as soon as threads become available ("non-FIFO" completion order). The global concurrent queues represent three priority bands:

       •   DISPATCH_QUEUE_PRIORITY_HIGH        •   DISPATCH_QUEUE_PRIORITY_DEFAULT        •   DISPATCH_QUEUE_PRIORITY_LOW 

Blocks submitted to the high priority global queue will be invoked before those submitted to the default or low priority global queues. Blocks submitted to the low priority global queue will only be invoked if no blocks are pending on the default or high priority queues.

So, they are queues which run on background threads as and when they become available. They're "non-FIFO" so ordering is not guaranteed.