dispatch_async vs. dispatch_sync using Serial Queues in Grand Central Dispatch

Question

OK, I love Grand Central Dispatch and after using it with relative success but this is something I don't fully understand.

Suppose I have created my own serial queue using

dispatch_queue_t myQueue;
myQueue = dispatch_queue_create("myQueue", NULL);

After that I do this:

dispatch_async(myQueue, ^{
  [self doStuff1];
});

// and a few lines later...

dispatch_sync(myQueue, ^{
  [self doStuff2];
});

The first dispatch is async. So, it will be done concurrently, right? How can that be if myQueue is serial? How can a serial queue do things in parallel or, if you will, out of order?

thanks

Answer 1

dispatch_async() means that the block is enqueued and dispatch_async()returns to enqueueing another task/block (possibly) prior to the block being executed.

With dispatch_sync(), the block is enqueued and the function will not continue enqueueing another task/block until the block is executed.

The blocks are still executed serially. You could execute 100 dispatch_async() calls, each with a block that sleeps for 100 seconds, and it'd be really fast. Follow that with a call to dispatch_sync() on the same serial queue and dispatch_sync() will return ~10,000 seconds later.

---

To put it more simply:

dispatch_async(serialQ, block1);
dispatch_async(serialQ, block2);
dispatch_sync(serialQ, block3);

block1 will be executed before block2 which will be executed before block3. That is the order guaranteed by the serial queue.

However, the calls to dispatch_async() may return before any of the blocks start executing. The dispatch_sync() will not return before all three blocks are executed!

Answer 2

Neither dispatch_async or dispatch_sync change the way the block gets queued. If the queue is serial, the blocks will execute in a serial manner, if the queue is concurrent, in a concurrent manner.

The important difference between the two is that dispatch_sync queues the block and waits on the current execution thread until that block is executed and dispatch_async just queues the block and continues the execution of the subsequent instructions.

Answer 3

Serial Queue can only run a single task at a time,irrespective of sync or async. Serial queues are allocated only one thread. This will be easier to understand using the below example -

Suppose there are 2 Queues A and B running tasks T1 and T2 respectively and T1 is being executed asynchronously. If control passes from A to B and B runs T2 synchronously then till the time the T2(block of code in the dispatch_sync block) completes execution T1 will be blocked. When T2 completes then T1 will resume it's execution.