Macos – Using grand central dispatch in Linux

It is a library, but there are some kernel optimizations to allow for system level control. In particular, what happens is that there is an addition interface pthread_workqueue that allows GCD to tell the kernel it wants a thread to run some particular function, but doesn't actually start a thread (it is basically a continuation). At that point the kernel can choose to start that continuation or not depending on system load.

So yes, there is a global system wide infrastructure that manages GCD threads in the kernel, and the second answer is the correct one. The mistake you are making is thinking that there is synchronization going on there that is going to cost something. The scheduler is going to run no matter what, what GCD has done is used a new interface that lets the scheduler not only decide whether or not to run the threads based on their relative priority, but whether or not to create or destroy the threads as well.

It is a (significant) optimization, but it is not strictly necessary, and the FreeBSD port doesn't actually have support for the system wide stuff. If you want to look at the actual interfaces, here is pthread_workqueue.h, the implementation is in Apple's pthread.c, and you can see the stub entry point the kernel uses for starting up the workqueues in their asm stubs in start_wqthread.s. You can also go crawling through xnu to see how it upcalls into the stub if you really want.

You use it when you want to execute a block and wait for the results.

One example of this is the pattern where you're using a dispatch queue instead of locks for synchronization. For example, assume you have a shared NSMutableArray a, with access mediated by dispatch queue q. A background thread might be appending to the array (async), while your foreground thread is pulling the first item off (synchronously):

NSMutableArray *a = [[NSMutableArray alloc] init];
// All access to `a` is via this dispatch queue!
dispatch_queue_t q = dispatch_queue_create("com.foo.samplequeue", NULL);

dispatch_async(q, ^{ [a addObject:something]; }); // append to array, non-blocking

__block Something *first = nil;            // "__block" to make results from block available
dispatch_sync(q, ^{                        // note that these 3 statements...
        if ([a count] > 0) {               // ...are all executed together...
             first = [a objectAtIndex:0];  // ...as part of a single block...
             [a removeObjectAtIndex:0];    // ...to ensure consistent results
        }
});