Grand Central Dispatches - execution out of order?

Question

I'm trying to get a better understanding of GCD so I wrote the test code below (Bottom). Essentially two functions with waits inside sent on different queues and println on main thread waiting for particular tasks.

I expect the console output should be:

BEFORE FUNCTIONS
start 3 sec loop
start 5 sec loop
end 3 sec loop
BETWEEN FUNCTIONS WAIT ON LONG
AFTER FUNCTIONS WAIT ON LONG
end 5 sec loop
BETWEEN FUNCTIONS WAIT ON LONGER
AFTER FUNCTIONS WAIT ON LONGER

but what I get instead is:

BEFORE FUNCTIONS
start 3 sec loop
start 5 sec loop
BETWEEN FUNCTIONS WAIT ON LONGER
AFTER FUNCTIONS WAIT ON LONGER
end 3 sec loop
BETWEEN FUNCTIONS WAIT ON LONG
AFTER FUNCTIONS WAIT ON LONG
end 5 sec loop

a number of things don't make sense to me - 1. Why are the "Longer" ones printed before the "Long"? 2. Why are the "Longer" ones not printed after the END of the longer i.e. 5 second function?

CODE (ViewController.swift)

import UIKit

class ViewController: UIViewController {
  
  var longQueue = dispatch_group_create()
  var longerQueue = dispatch_group_create()
  var queueLow = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_LOW, 0)
  var queueHigh = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0)
  
  override func didReceiveMemoryWarning() {
    super.didReceiveMemoryWarning()
  }
  
  override func viewDidLoad() {
    super.viewDidLoad()

    println("BEFORE FUNCTIONS")
    
    dispatch_async(queueHigh) {
      self.longFunc()
    }
    
    dispatch_group_notify(longQueue, dispatch_get_main_queue()) {
      println("BETWEEN FUNCTIONS WAIT ON LONG")
    }
    dispatch_group_notify(longerQueue, dispatch_get_main_queue()) {
      println("BETWEEN FUNCTIONS WAIT ON LONGER")
    }
    
    dispatch_async(queueLow) {
      self.longerFunc()
    }

    dispatch_group_notify(longQueue, dispatch_get_main_queue()) {
      println("AFTER FUNCTIONS WAIT ON LONG")
    }
    dispatch_group_notify(longerQueue, dispatch_get_main_queue()) {
      println("AFTER FUNCTIONS WAIT ON LONGER")
    }
  }
  
  func longFunc () {
    dispatch_group_enter(self.longQueue)
      println("start 3 sec loop")
      sleep(3)
      println("end 3 sec loop")
    dispatch_group_leave(self.longQueue)
  }
  
  func longerFunc() {
    dispatch_group_enter(self.longerQueue)
      println("start 5 sec loop")
      sleep(5)
      println("end 5 sec loop")
    dispatch_group_leave(self.longerQueue)
  }

}

Rob · Accepted Answer

There are two categories of problems here:

You are entering the dispatch groups inside the longFunc and longerFunc. But, because you're dispatching the calls to those functions to their respective queues, you have no assurances that you're going to reach dispatch_group_enter before you reach the dispatch_group_notify calls you perform in viewDidLoad on the main thread. Also remember that the code that is dispatched to the background queues runs asynchronously and we have no assurances when it runs versus the code that is continuing to execute on the main thread. The key is that if it encounters dispatch_group_notify before you've even entered the group, that notify closure will fire immediately.

To avoid this race condition, you either want to perform the dispatch_group_enter in viewDidLoad before you dispatch the code to the background queues (and obviously, before you set up the notify closure), or you want to just retire the entering and leaving of the groups and then replace dispatch_async with dispatch_group_async. Either of those approaches ensure that you enter groups before you add the notify closures to those groups.
The dispatch notify closure that reports "BETWEEN FUNCTIONS WAIT ON LONGER" is an even more egregious example, where there's not even a race condition. You are simply adding that notify closure before you even dispatch the function that will later enter the group. So this will almost certainly fire before you code is dispatched to that queue.

Grand Central Dispatches - execution out of order?

CODE (ViewController.swift)

Answers (2)

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