On Apr 19, 2011, at 9:09 AM, Dave Zarzycki wrote:

On Apr 19, 2011, at 8:07 AM, Marius Zwicker wrote:

I thought the main queue is available as a mean to be sure some blocks execute on the main thread (e.g. when updating the GUI is only possible while being on the main thread).

Marius,

That is only true if-and-only-if GCD is working in compatibility mode with Cocoa GUI logic, which for legacy reasons is bound to the main thread. If dispatch_main() is called, then GCD turns the main queue into a normal serial queue, with all of the performance advantages thereof.

P.S. – This is covered in the dispatch_main() man page:

COMPATIBILITY

     Cocoa applications need not call dispatch_main().  Blocks submitted to

     the main queue will be executed as part of the "common modes" of the

     application's main NSRunLoop or CFRunLoop.  However, blocks submitted to

     the main queue in applications using dispatch_main() are not guaranteed

     to execute on the main thread.