I'm writing a moderately complex iOS program that needs to have multiple threads for some of its longer operations (parsing, connections to the network...etc). However, I'm confused as to what the difference is between
As the documentation describes, a global queue is good for concurrent tasks (i.e. you're going to dispatch various tasks asynchronously and you're perfectly happy if they run concurrently) and if you don't want to encounter the theoretical overhead of creating and destroying your own queue.
The creating of your own queue is very useful if you need a serial queue (i.e. you need the dispatched blocks to be executed one at a time). This can be useful in many scenarios, such as when each task is dependent upon the preceding one or when coordinating interaction with some shared resource from multiple threads.
Less common, but you will also want to create your own queue if you need to use barriers in conjunction with a concurrent queue. In that scenario, create a concurrent queue (i.e.
dispatch_queue_create with the
DISPATCH_QUEUE_CONCURRENT option) and use the barriers in conjunction with that queue. You should never use barriers on global queues.
My general counsel is if you need a serial queue (or need to use barriers), then create a queue. If you don't, go ahead and use the global queue and bypass the overhead of creating your own.
If you want a concurrent queue, but want to control how many operations can run concurrently, you can also consider using
NSOperationQueue which has a
maxConcurrentOperationCount property. This can be useful when doing network operations and you don't want too many concurrent requests being submitted to your server.