Concurrent vs serial queues in GCD

I'm struggling to fully understand the concurrent and serial queues in GCD. I have some issues and hoping someone can answer me clearly and at the point.

I'm reading that serial queues are created and used in order to execute tasks one after the other. However, what happens if:
- I create a serial queue
- I use dispatch_async (on the serial queue I just created) three times to dispatch three blocks A,B,C
Will the three blocks be executed:
- in order A,B,C because the queue is serial
  
  OR
- concurrently (in the same time on parralel threads) because I used ASYNC dispatch
I'm reading that I can use dispatch_sync on concurrent queues in order to execute blocks one after the other. In that case, WHY do serial queues even exist, since I can always use a concurrent queue where I can dispatch SYNCHRONOUSLY as many blocks as I want?

Thanks for any good explanation!

Solution 1:

A simple example: you have a block that takes a minute to execute. You add it to a queue from the main thread. Let's look at the four cases.

async - concurrent: the code runs on a background thread. Control returns immediately to the main thread (and UI). The block can't assume that it's the only block running on that queue
async - serial: the code runs on a background thread. Control returns immediately to the main thread. The block can assume that it's the only block running on that queue
sync - concurrent: the code runs on a background thread but the main thread waits for it to finish, blocking any updates to the UI. The block can't assume that it's the only block running on that queue (I could have added another block using async a few seconds previously)
sync - serial: the code runs on a background thread but the main thread waits for it to finish, blocking any updates to the UI. The block can assume that it's the only block running on that queue

Obviously you wouldn't use either of the last two for long running processes. You normally see it when you're trying to update the UI (always on the main thread) from something that may be running on another thread.

Solution 2:

Here are a couple of experiments that i have done to make me understand about these serial, concurrent queues with Grand Central Dispatch.

 func doLongAsyncTaskInSerialQueue() {

   let serialQueue = DispatchQueue(label: "com.queue.Serial")
      for i in 1...5 {
        serialQueue.async {

            if Thread.isMainThread{
                print("task running in main thread")
            }else{
                print("task running in background thread")
            }
            let imgURL = URL(string: "https://upload.wikimedia.org/wikipedia/commons/0/07/Huge_ball_at_Vilnius_center.jpg")!
            let _ = try! Data(contentsOf: imgURL)
            print("\(i) completed downloading")
        }
    }
}

Task will run in different thread(other than main thread) when you use async in GCD. Async means execute next line do not wait until the block executes which results non blocking main thread & main queue. Since its serial queue, all are executed in the order they are added to serial queue.Tasks executed serially are always executed one at a time by the single thread associated with the Queue.

func doLongSyncTaskInSerialQueue() {
    let serialQueue = DispatchQueue(label: "com.queue.Serial")
    for i in 1...5 {
        serialQueue.sync {
            if Thread.isMainThread{
                print("task running in main thread")
            }else{
                print("task running in background thread")
            }
            let imgURL = URL(string: "https://upload.wikimedia.org/wikipedia/commons/0/07/Huge_ball_at_Vilnius_center.jpg")!
            let _ = try! Data(contentsOf: imgURL)
            print("\(i) completed downloading")
        }
    }
}

Task may run in main thread when you use sync in GCD. Sync runs a block on a given queue and waits for it to complete which results in blocking main thread or main queue.Since the main queue needs to wait until the dispatched block completes, main thread will be available to process blocks from queues other than the main queue.Therefore there is a chance of the code executing on the background queue may actually be executing on the main thread Since its serial queue, all are executed in the order they are added(FIFO).

func doLongASyncTaskInConcurrentQueue() {
    let concurrentQueue = DispatchQueue(label: "com.queue.Concurrent", attributes: .concurrent)
    for i in 1...5 {
        concurrentQueue.async {
            if Thread.isMainThread{
                print("task running in main thread")
            }else{
                print("task running in background thread")
            }
            let imgURL = URL(string: "https://upload.wikimedia.org/wikipedia/commons/0/07/Huge_ball_at_Vilnius_center.jpg")!
            let _ = try! Data(contentsOf: imgURL)
            print("\(i) completed downloading")
        }
        print("\(i) executing")
    }
}

Task will run in background thread when you use async in GCD. Async means execute next line do not wait until the block executes which results non blocking main thread. Remember in concurrent queue, task are processed in the order they are added to queue but with different threads attached to the queue. Remember they are not supposed to finish the task as the order they are added to the queue.Order of task differs each time threads are created as necessarily automatically.Task are executed in parallel. With more than that(maxConcurrentOperationCount) is reached, some tasks will behave as a serial until a thread is free.

func doLongSyncTaskInConcurrentQueue() {
  let concurrentQueue = DispatchQueue(label: "com.queue.Concurrent", attributes: .concurrent)
    for i in 1...5 {
        concurrentQueue.sync {
            if Thread.isMainThread{
                print("task running in main thread")
            }else{
                print("task running in background thread")
            }
            let imgURL = URL(string: "https://upload.wikimedia.org/wikipedia/commons/0/07/Huge_ball_at_Vilnius_center.jpg")!
            let _ = try! Data(contentsOf: imgURL)
            print("\(i) completed downloading")
        }
        print("\(i) executed")
    }
}

Task may run in main thread when you use sync in GCD. Sync runs a block on a given queue and waits for it to complete which results in blocking main thread or main queue.Since the main queue needs to wait until the dispatched block completes, main thread will be available to process blocks from queues other than the main queue.Therefore there is a chance of the code executing on the background queue may actually be executing on the main thread. Since its concurrent queue, tasks may not finish in the order they are added to queue. But with synchronous operation it does although they may be processed by different threads. So, it behaves as this is the serial queue.

Here is a summary of these experiments

Remember using GCD you are only adding task to the Queue and performing task from that queue. Queue dispatches your task either in main or background thread depending on whether operation is synchronous or asynchronous. Types of queues are Serial,Concurrent,Main dispatch queue.All the task you perform is done by default from Main dispatch queue.There are already four predefined global concurrent queues for your application to use and one main queue(DispatchQueue.main).You can also manually create your own queue and perform task from that queue.

UI Related task should always be performed from main thread by dispatching the task to Main queue.Short hand utility is DispatchQueue.main.sync/async whereas network related/heavy operations should always be done asynchronously no matters which ever thread you are using either main or background

EDIT: However, There are cases you need to perform network calls operations synchronously in a background thread without freezing UI(e.g.refreshing OAuth Token and wait if it succeed or not).You need to wrap that method inside a asynchronous operation.This way your heavy operations are executed in the order and without Blocking main thread.

func doMultipleSyncTaskWithinAsynchronousOperation() {
    let concurrentQueue = DispatchQueue(label: "com.queue.Concurrent", attributes: .concurrent)
    concurrentQueue.async {
        let concurrentQueue = DispatchQueue.global(qos: DispatchQoS.QoSClass.default)
        for i in 1...5 {
            concurrentQueue.sync {
                let imgURL = URL(string: "https://upload.wikimedia.org/wikipedia/commons/0/07/Huge_ball_at_Vilnius_center.jpg")!
                let _ = try! Data(contentsOf: imgURL)
                print("\(i) completed downloading")
            }
            print("\(i) executed")
        }
    }
}

EDIT EDIT: You can watch demo video here

Concurrent vs serial queues in GCD

Solution 1:

Solution 2:

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