Does dispatch_async block to serial queue keeps order?

Question

When I submit two blocks into a serial queue by dispatch_async, does it assure the second one runs after the first one:

dispatch_async(serial_queue, b1);
dispatch_async(serial_queue, b2);

can we assure b1 runs before b2?

Below is full source code section:



    #define COUNTER 10000
      m_value = 0;
      dispatch_queue_t globalQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
      dispatch_queue_t queue = dispatch_queue_create("myqueue", NULL);
      dispatch_group_t group = dispatch_group_create();
       for (int i = 0; i &lt COUNTER; ++i) {
        dispatch_group_async(group, globalQueue, ^() {
          dispatch_async(queue, ^() {
            ++m_value;
          });
        });
      }
      dispatch_group_notify(group, queue, ^() {
        NSLog(@"m_value Actual: %d Expected: %d", m_value, COUNTER);
      });

      dispatch_release(queue);
      dispatch_release(group);
      queue = nil;
      group = nil;
      return YES;

Can we assure m_value == COUNTER always? Thanks

score 3 · Answer 1 · answered Oct 16 '12 at 18:21

3

Blocks submitted to concurrent queues may execute concurrently, however from the Apple docs "Blocks submitted to a serial queue are executed one at a time in FIFO order." The main queue is serial as are any queues that you create with dispatch_create_queue.

answered Oct 16 '12 at 18:21

Thomas Bouldin

3,707
19
21

Awesome, so if we want FIFO we can simply use `dispatch_create_queue`, and set the priority with `dispatch_set_target_queue` [as per this answer](http://stackoverflow.com/a/9965389/72176) :) – William Denniss Nov 24 '12 at 03:43
Yes, though I'm not sure you need dispatch_set_target_queue. I rarely see a case where it is correct for a developer to *increase* priority. – Thomas Bouldin Nov 26 '12 at 18:22
I used it to _decrease_ priority to the lowest level. I was processing photos using a queue created with `dispatch_create_queue`, and it seemed to help (UI was more responsive). – William Denniss Nov 27 '12 at 01:01
1

In this case, you might consider the background queue where available (4.3.0 and greater). This is guaranteed to be serial and will wait until foreground activity has settled. – Thomas Bouldin Dec 02 '12 at 23:55
Is ```dispatch_queue_create``` instead of ```dispatch_create_queue```? – Lubbo Feb 27 '18 at 10:43

score 0 · Answer 2 · answered Jul 03 '12 at 18:10

0

according the documentation the blocks (on global queue) could be executed concurrently

"Blocks submitted to these global concurrent queues may be executed concurrently with respect to each other."

answered Jul 03 '12 at 18:10

Sandeep Chayapathi

1,490
2
13
31

Does dispatch_async block to serial queue keeps order?

2 Answers2