iOS GCD (Grand Central Dispatch) Serial vs Concurrent Dispatch Queues
Example
Swift 3
Serial Queue
func serialQueues () {
let serialQueue = DispatchQueue(label: "com.example.serial") //default queue type is a serial queue
let start = Date ()
for i in 0...3 { //launch a bunch of tasks
serialQueue.async { //run tasks on a background thread, using our serial queue
sleep(2) //do some long task eg webservice or database lookup
let timeTaken = Date().timeIntervalSince(start)
print("serial long task \(i) done! total time taken: \(timeTaken)")
}
}
}
Example output:
serial long task 0 done! total time taken: 2.07241100072861
serial long task 1 done! total time taken: 4.16347700357437
serial long task 2 done! total time taken: 6.23209798336029
serial long task 3 done! total time taken: 8.30682599544525
Concurrent Queue
func concurrentQueues () {
let concurrentQueue = DispatchQueue(label: "com.example.concurrent", attributes: .concurrent) //explicitly specify the queue to be a concurrent queue
let start = Date ()
for i in 0...3 { //launch a bunch of tasks
concurrentQueue.async { //run tasks on a background thread, using our concurrent queue
sleep(2) //do some long task eg webservice or database lookup
let timeTaken = Date().timeIntervalSince(start)
print("concurrent long task \(i) done! total time taken: \(timeTaken)")
}
}
}
Example output:
concurrent long task 3 done! total time taken: 2.07092100381851
concurrent long task 0 done! total time taken: 2.07087397575378
concurrent long task 2 done! total time taken: 2.07086700201035
concurrent long task 1 done! total time taken: 2.07089096307755
Discussion
As we can see from the examples above, a serial queue will complete each task in the order they are submitted to the queue. Each task will wait for the previous task to finish before executing. As for the concurrent queue, each task does not wait on the others in the queue and executes as soon as possible; the advantage is that all tasks on the queue will run at the same time on separate threads, making a concurrent queue take less time than a serial queue.
If order of execution of tasks is not important, always use a concurrent queue for the best efficiency.
