GCD Part 2: DispatchWorkItem and Quality of Service

Alex Shchukin
6 min readJul 2, 2021

This is the second part of the GCD series and here we will discuss QoS and DispatchWorkItem.

DispatchWorkItem

There is a way to add a task to the queue through a special class called DispatchWorkItem instead of direct passing the closure to async or sync methods. This class provides additional methods to interact with the task. For example, sometimes it is necessary to receive a completion notification. In that case, we need to call method notify and pass the completion block. We also need to specify in which queue (in the example below it’s the main queue) the completion will be executed.

let item = DispatchWorkItem {
print(“test”)
}
item.notify(queue: DispatchQueue.main) {
print(“finish”)
}
serialQueue.async(execute: item)

Result:

test
finish

We can also execute DispatchWorkItem manually using perform method:

let workItem = DispatchWorkItem {
print(“test”)
}
workItem.perform()

Another useful case for DispatchWorkItem is the ability to cancel tasks through the cancel method in DispatchWorkItem. But there is a limitation: the cancellation will work only if the task has not started yet. Let’s how it works in the example below:

serialQueue.async {
print(“test1”)
sleep(1)
}
serialQueue.async {
print(“test2”)
sleep(1)
}
let item = DispatchWorkItem {
print(“test”)
}
serialQueue.async(execute: item)item.cancel()

Result:

test1
<- 1 second wait time ->
test2

Method wait is a very useful method. It blocks the calling thread until DispatchWorkItem finishes its task. Remember that’s not a good idea to call the method wait on the main thread. DispatchGroup has similar functionality and we will discuss it in the next article.

let workItem = DispatchWorkItem {
print(“test1”)
sleep(1)
}
serialQueue.async(execute: workItem)
workItem.wait()
print(“test2”)

Result:

test1
<- 1 second wait time ->
test2

There are plenty of flags you can set in the init of DispatchWorkItem most of them related to QoS but one can be considered out of the QoS context. It’s called barrier and it’s actually pretty similar to other barriers functionality we consider in this series. The key idea is that the work item is created with this parameter and added to the concurrent queue will wait until all the tasks in that queue will be finished and will block the execution of others until it will not finish. For a better understanding let’s check how it works in the example below:

let concurrentQueue = DispatchQueue(label: “com.test.concurrent”, attributes: .concurrent)let workItem = DispatchWorkItem(flags: .barrier) {
print(“test2”)
sleep(3)
}
concurrentQueue.async {
print(“test1”)
sleep(3)
}
concurrentQueue.async(execute: workItem)concurrentQueue.async {
print(“test3”)
}

Result:

test1
<- 3 seconds ->
test2
<- 3 seconds ->
test3

QoS

In modern apps, we as developers usually try to find some balance between performance and battery usage. Since we work in a concurrent environment we need to prioritize some of our tasks based on their importance. For example, the user clicks a button and an animation should be displayed. In that case, we want a high prioritization of the rendering task. Or another example, we want to run some cleanup task of removing temporary files and the user shouldn’t receive any updates from this task so we can say that is a low-prioritized issue.

Quality of service is a single abstract parameter you can use to classify your work by its importance. There are four types of quality of service: userInteractive, userInitiated, utility and background. For the high-priority task, the application spends much more energy since it consumes more resources and for the low priority task, it spends lower energy.

userInteractive — for tasks based on the user interaction like the refreshing user interface or performing rendering. The main thread of the application always comes with userInteractive mode.

userInitiated — for tasks initiated by the user and required immediate result like the user clicks on the UI element and expects a quick response.

utility — for tasks doesn’t require immediate result but the user needs to be updated like downloading task with the progress bar.

background — for tasks that are not visible to the user like synchronizing or cleaning tasks.

There are two additional QoS classes default and unspecified that developers should not use directly

default — according to Apple documentation, the priority level of this QoS is between userInitiated and utility.

unspecified — means the absence of information about QoS and expects it will be propagated (we will explore this in the next paragraph).

Interesting fact, for the global queue, if we don’t specify QoS the value should be default:

class func global(qos: DispatchQoS.QoSClass = .default) -> DispatchQueue

But in fact, it has unspecified value:

print(DispatchQueue.global().qos.qosClass)
print(DispatchQueue.global(qos: .background).qos.qosClass)

Result:

unspecified
background

QoS propagation

Another important thing to understand is how QoS can be propagated between queues. As I mentioned before the main thread is associated with userInteractive value. That means all the tasks you are executing on the main thread will take the highest priority.

DispatchQueue.main.async {
print(DispatchQoS.QoSClass(rawValue: qos_class_self()) ?? .unspecified)
}

Result:

userInteractive

In case when we don’t specify QoS directly in the queue it acquires the QoS from the calling thread. As you can see in the example below we don’t specify the QoS for the serial queue and it captures it automatically from the calling utility queue. This mechanic is called automatic propagation.

let serialQueue = DispatchQueue(label: “com.test.serial”)
let utilityQueue = DispatchQueue(label: “com.test.utility”, qos: .utility)
utilityQueue.async {
serialQueue.async {
print(DispatchQoS.QoSClass(rawValue: qos_class_self()) ?? .unspecified)
}
}

Result:

utility

There is one important exception for the previous rule — if we add the task to the queue from the userInteractive thread (or main thread) it automatically drops from userInteractive to userInitiated.

DispatchQueue.main.async {
serialQueue.async {
print(DispatchQoS.QoSClass(rawValue: qos_class_self()) ?? .unspecified)
}
}

Result:

userInitiated

Also, this rule doesn’t work backward. It means if we call from the low-priority thread the high-priority task it keeps its own high priority. In the example below the calling queue (utilityQueue) has low priority compared to the called queue (userInitiatedQueue) so the task of the called queue is to be executed in userInitiated mode.

utilityQueue.async {
print(DispatchQoS.QoSClass(rawValue: qos_class_self()) ?? .unspecified)
userInitiatedQueue.async {
print(DispatchQoS.QoSClass(rawValue: qos_class_self()) ?? .unspecified)
}
}

Result:

utility
userInitiated

Let’s consider the case when we need directly to specify the QoS of the executing task. To do that we can set QoS as a parameter for async or sync methods of the serialQueue we created before. Or we can associate the queue with a specific QoS and set it as a parameter on its creation.

let serialQueue = DispatchQueue(label: “com.test.serial”)serialQueue.async(qos: .utility) {
print(DispatchQoS.QoSClass(rawValue: qos_class_self()) ?? .unspecified)
}
// Orlet utilityQueue = DispatchQueue(label: “com.test.utility”, qos: .utility)utilityQueue.async {
print(DispatchQoS.QoSClass(rawValue: qos_class_self()) ?? .unspecified)
}

Result:

utility

Ok now we know how to use QoS with the queues but there are more sophisticated cases with DispatchWorkItem. Using the flags parameter in the init we can define how QoS will be propagated to the task (or not). The first flag we consider is called inheritQoS it means that the executed task will prefer to assign QoS from the calling thread.

utilityQueue.async {let workItem = DispatchWorkItem(qos: .userInitiated, flags: .inheritQoS) {print(DispatchQoS.QoSClass(rawValue: qos_class_self()) ?? .unspecified)}workItem.perform()}// Orlet workItem = DispatchWorkItem(qos: .userInitiated, flags: .inheritQoS) {print(DispatchQoS.QoSClass(rawValue: qos_class_self()) ?? .unspecified)}utilityQueue.async(execute: workItem)

Result:

utility

Another flag is called enforceQoS and has reverse functionality with the previous one. In this case, the task will acquire QoS from the DispatchWorkItem.

let workItem = DispatchWorkItem(qos: .userInitiated, flags: .enforceQoS) {
print(DispatchQoS.QoSClass(rawValue: qos_class_self()) ?? .unspecified)
}
utilityQueue.async(execute: workItem)

Result:

userInitiated

There is one important addition to that functionality. Let’s say we have a serial queue and its QoS is utility and there is already a task added to the queue (since it doesn’t have any flags it also has QoS utility). This situation causes the Priority Inversion and GCD automatically resolves it raising the QoS of the low-prioritized task. That is not visible to the developer since it’s caused by GCD. But ofc we need to keep it in mind by developing concurrent applications.

utilityQueue.async {
sleep(2)
}
let workItem = DispatchWorkItem(qos: .userInitiated, flags: .enforceQoS) {
sleep(1)
}
utilityQueue.async(execute: workItem)

So in this part, we discussed pretty complicated moments related to QoS. In the next article, we will look at DispatchGroup and ways to work with it.

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