I have a thread setup where I call a setup() function:
let queue = DispatchQueue(label: "my-queue", qos: .utility)
queue.async {
self.setup {
}
}
If the .async block is triggered multiple times, I want the setup() function to be called either:
multiple times, just not at the same time.
or
one time, with previously unfinished calls being cancelled, and only the last call of the function returning.
Either is acceptable in my case.
Is there a swifty way to accomplish this or is the only solution RxSwift?
let serialQueue = DispatchQueue(label: "Serial Queue")
func performCriticalSectionTask() {
serialQueue.async {
performLongRuningAsyncTask()
}
}
func performLongRuningAsyncTask() {
/// some long running task
}
The function performCriticalSectionTask() can be called from different places many times.
I want this function to be running one at a time. Thus, I kept the critical section of code inside the serial async queue.
But, the problem here is that the critical section itself is a performLongRuningAsyncTask() which will return immediately, and thus serial queue will not wait for the current task to complete first and will start another one.
How can I solve this problem?
if performLongRuningAsyncTask is only running in one thread, it will be called only once at the time. In your case it delegates it to another thread, so you wrapping it into another thread call doesn't work since it will be on another thread anyway
You could do checks in the method itself, the simplest way is to add a boolean. (Or you could add these checks in your class that executes this method, with a completion handler).
Another ways are adding dispatch groups / semaphores / locks.
If you still need it to be executed later, you should use a dispatch group / OperationQueue / Semaphore.
func performLongRunningAsyncTask() {
self.serialQueue.sync {
if isAlreadyRunning {
return
}
isAlreadyRunning = true
}
asyncTask { result in
self.serialQueue.sync {
self.isAlreadyRunning = false
}
}
}
In my app i have a task with a lot of mathematics. If i run this task in main queue, i have frozen for a few seconds screen after each call of task, but it work. If i run task in other queue - from some random iteration it do nothing. If i run code in main queue, i get debug message in every iteration, in other queue - i get no one after random iteration. Looks like, the queue dying for some reasons. Usage of cpu and memory doesn't change and stay at level 50-70%. I think about endless loop, deadlock or something similar in function, but in main queue it always work fine. What goes wrong?
class MyClass {
let serialQueue = DispatchQueue(
label: "com.notrealcompany.hardMathematics",
qos: .userInteractive
)
func doStuff() {
serialQueue.async {
node.getArea()
debugPrint("get area call")
}
}
serialQueue is an instance variable, but situation don't change.
It sounds like serialQueue is being deallocated when the method your code is in returns. Try moving serialQueue's declaration to an instance variable instead of a local variable.
class MyClass {
let serialQueue = DispatchQueue(
label: "com.notrealcompany.hardMathematics",
qos: .userInteractive
)
func doStuff() {
serialQueue.async {
node.getArea()
debugPrint("get area call")
}
}
}
I would assume that I am aware of how to work with DispatchGroup, for understanding the issue, I've tried:
class ViewController: UIViewController {
override func viewDidLoad() {
super.viewDidLoad()
performUsingGroup()
}
func performUsingGroup() {
let dq1 = DispatchQueue.global(qos: .userInitiated)
let dq2 = DispatchQueue.global(qos: .userInitiated)
let group = DispatchGroup()
group.enter()
dq1.async {
for i in 1...3 {
print("\(#function) DispatchQueue 1: \(i)")
}
group.leave()
}
group.wait()
dq2.async {
for i in 1...3 {
print("\(#function) DispatchQueue 2: \(i)")
}
}
group.notify(queue: DispatchQueue.main) {
print("done by group")
}
}
}
and the result -as expected- is:
performUsingGroup() DispatchQueue 1: 1
performUsingGroup() DispatchQueue 1: 2
performUsingGroup() DispatchQueue 1: 3
performUsingGroup() DispatchQueue 2: 1
performUsingGroup() DispatchQueue 2: 2
performUsingGroup() DispatchQueue 2: 3
done by group
For using the Semaphore, I implemented:
func performUsingSemaphore() {
let dq1 = DispatchQueue.global(qos: .userInitiated)
let dq2 = DispatchQueue.global(qos: .userInitiated)
let semaphore = DispatchSemaphore(value: 1)
dq1.async {
semaphore.wait()
for i in 1...3 {
print("\(#function) DispatchQueue 1: \(i)")
}
semaphore.signal()
}
dq2.async {
semaphore.wait()
for i in 1...3 {
print("\(#function) DispatchQueue 2: \(i)")
}
semaphore.signal()
}
}
and called it in the viewDidLoad method. The result is:
performUsingSemaphore() DispatchQueue 1: 1
performUsingSemaphore() DispatchQueue 1: 2
performUsingSemaphore() DispatchQueue 1: 3
performUsingSemaphore() DispatchQueue 2: 1
performUsingSemaphore() DispatchQueue 2: 2
performUsingSemaphore() DispatchQueue 2: 3
Conceptually, both of DispachGroup and Semaphore serve the same purpose (unless I misunderstand something).
Honestly, I am unfamiliar with: when to use the Semaphore, especially when workin with DispachGroup -probably- handles the issue.
What is the part that I am missing?
Conceptually, both of DispatchGroup and Semaphore serve the same purpose (unless I misunderstand something).
The above is not exactly true. You can use a semaphore to do the same thing as a dispatch group but it is much more general.
Dispatch groups are used when you have a load of things you want to do that can all happen at once, but you need to wait for them all to finish before doing something else.
Semaphores can be used for the above but they are general purpose synchronisation objects and can be used for many other purposes too. The concept of a semaphore is not limited to Apple and can be found in many operating systems.
In general, a semaphore has a value which is a non negative integer and two operations:
wait If the value is not zero, decrement it, otherwise block until something signals the semaphore.
signal If there are threads waiting, unblock one of them, otherwise increment the value.
Needless to say both operations have to be thread safe. In olden days, when you only had one CPU, you'd simply disable interrupts whilst manipulating the value and the queue of waiting threads. Nowadays, it is more complicated because of multiple CPU cores and on chip caches etc.
A semaphore can be used in any case where you have a resource that can be accessed by at most N threads at the same time. You set the semaphore's initial value to N and then the first N threads that wait on it are not blocked but the next thread has to wait until one of the first N threads has signaled the semaphore. The simplest case is N = 1. In that case, the semaphore behaves like a mutex lock.
A semaphore can be used to emulate a dispatch group. You start the sempahore at 0, start all the tasks - tracking how many you have started and wait on the semaphore that number of times. Each task must signal the semaphore when it completes.
However, there are some gotchas. For example, you need a separate count to know how many times to wait. If you want to be able to add more tasks to the group after you have started waiting, the count can only be updated in a mutex protected block and that may lead to problems with deadlocking. Also, I think the Dispatch implementation of semaphores might be vulnerable to priority inversion. Priority inversion occurs when a high priority thread waits for a resource that a low priority has grabbed. The high priority thread is blocked until the low priority thread releases the resource. If there is a medium priority thread running, this may never happen.
You can pretty much do anything with a semaphore that other higher level synchronisation abstractions can do, but doing it right is often a tricky business to get right. The higher level abstractions are (hopefully) carefully written and you should use them in preference to a "roll your own" implementation with semaphores, if possible.
Semaphores and groups have, in a sense, opposite semantics. Both maintain a count. With a semaphore, a wait is allowed to proceed when the count is non-zero. With a group, a wait is allowed to proceed when the count is zero.
A semaphore is useful when you want to set a maximum on the number of threads operating on some shared resource at a time. One common use is when the maximum is 1 because the shared resource requires exclusive access.
A group is useful when you need to know when a bunch of tasks have all been completed.
Use a semaphore to limit the amount of concurrent work at a given time. Use a group to wait for any number of concurrent work to finish execution.
In case you wanted to submit three jobs per queue it should be
import Foundation
func performUsingGroup() {
let dq1 = DispatchQueue(label: "q1", attributes: .concurrent)
let dq2 = DispatchQueue(label: "q2", attributes: .concurrent)
let group = DispatchGroup()
for i in 1...3 {
group.enter()
dq1.async {
print("\(#function) DispatchQueue 1: \(i)")
group.leave()
}
}
for i in 1...3 {
group.enter()
dq2.async {
print("\(#function) DispatchQueue 2: \(i)")
group.leave()
}
}
group.notify(queue: DispatchQueue.main) {
print("done by group")
}
}
performUsingGroup()
RunLoop.current.run(mode: RunLoop.Mode.default, before: Date(timeIntervalSinceNow: 1))
and
import Foundation
func performUsingSemaphore() {
let dq1 = DispatchQueue(label: "q1", attributes: .concurrent)
let dq2 = DispatchQueue(label: "q2", attributes: .concurrent)
let semaphore = DispatchSemaphore(value: 1)
for i in 1...3 {
dq1.async {
_ = semaphore.wait(timeout: DispatchTime.distantFuture)
print("\(#function) DispatchQueue 1: \(i)")
semaphore.signal()
}
}
for i in 1...3 {
dq2.async {
_ = semaphore.wait(timeout: DispatchTime.distantFuture)
print("\(#function) DispatchQueue 2: \(i)")
semaphore.signal()
}
}
}
performUsingSemaphore()
RunLoop.current.run(mode: RunLoop.Mode.default, before: Date(timeIntervalSinceNow: 1))
The replies above by Jano and Ken are correct regarding 1) the use of semaphore to limit the amount of work happening at once 2) the use of a dispatch group so that the group will be notified when all the tasks in the group are done. For example, you may want to download a lot of images in parallel but since you know that they are heavy images, you want to limit to two downloads only at a single time so you use a semaphore. You also want to be notified when all the downloads (say there are 50 of them) are done, so you use DispatchGroup. Thus, it is not a matter of choosing between the two. You may use one or both in the same implementation depending on your goals. This type of example was provided in the Concurrency tutorial on Ray Wenderlich's site:
let group = DispatchGroup()
let queue = DispatchQueue.global(qos: .utility)
let semaphore = DispatchSemaphore(value: 2)
let base = "https://yourbaseurl.com/image-id-"
let ids = [0001, 0002, 0003, 0004, 0005, 0006, 0007, 0008, 0009, 0010, 0011, 0012]
var images: [UIImage] = []
for id in ids {
guard let url = URL(string: "\(base)\(id)-jpeg.jpg") else { continue }
semaphore.wait()
group.enter()
let task = URLSession.shared.dataTask(with: url) { data, _, error in
defer {
group.leave()
semaphore.signal()
}
if error == nil,
let data = data,
let image = UIImage(data: data) {
images.append(image)
}
}
task.resume()
}
One typical semaphore use case is a function that can be called simultaneously from different threads and uses a resource that should not be called from multiple threads at the same time:
func myFunction() {
semaphore.wait()
// access the shared resource
semaphore.signal()
}
In this case you will be able to call myFunction from different threads but they won't be able to reach the locked resource simultaneously. One of them will have to wait until the second one finishes its work.
A semaphore keeps a count, therefore you can actually allow for a given number of threads to enter your function at the same time.
Typical shared resource is the output to a file.
A semaphore is not the only way to solve such problems. You can also add the code to a serial queue, for example.
Semaphores are low level primitives and most likely they are used a lot under the hood in GCD.
Another typical example is the producer-consumer problem, where the signal and wait calls are actually part of two different functions. One which produces data and one which consumes them.
Generally semaphore can be considered mainly that we can solve the critical section problem. Locking certain resource to achieve synchronisation. Also what happens if sleep() is invoked, can we achieve the same thing by using a semaphore ?
Dispatch groups we will use when we have multiple group of operations to be carried out and we need a tracking or set dependencies each other or notification when a group os tasks finishes its execution.
I have a shared-memory between multiple threads. I want to prevent these threads access this piece of memory at a same time. (like producer-consumer problem)
Problem:
A thread add elements to a queue and another thread reads these elements and delete them. They shouldn't access the queue simultaneously.
One solution to this problem is to use Mutex.
As I found, there is no Mutex in Swift. Is there any alternatives in Swift?
There are many solutions for this but I use serial queues for this kind of action:
let serialQueue = DispatchQueue(label: "queuename")
serialQueue.sync {
//call some code here, I pass here a closure from a method
}
Edit/Update: Also for semaphores:
let higherPriority = DispatchQueue.global(qos: .userInitiated)
let lowerPriority = DispatchQueue.global(qos: .utility)
let semaphore = DispatchSemaphore(value: 1)
func letUsPrint(queue: DispatchQueue, symbol: String) {
queue.async {
debugPrint("\(symbol) -- waiting")
semaphore.wait() // requesting the resource
for i in 0...10 {
print(symbol, i)
}
debugPrint("\(symbol) -- signal")
semaphore.signal() // releasing the resource
}
}
letUsPrint(queue: lowerPriority, symbol: "Low Priority Queue Work")
letUsPrint(queue: higherPriority, symbol: "High Priority Queue Work")
RunLoop.main.run()
Thanks to beshio's comment, you can use semaphore like this:
let semaphore = DispatchSemaphore(value: 1)
use wait before using the resource:
semaphore.wait()
// use the resource
and after using release it:
semaphore.signal()
Do this in each thread.
As people commented (incl. me), there are several ways to achieve this kind of lock. But I think dispatch semaphore is better than others because it seems to have the least overhead. As found in Apples doc, "Replacing Semaphore Code", it doesn't go down to kernel space unless the semaphore is already locked (= zero), which is the only case when the code goes down into the kernel to switch the thread. I think that semaphore is not zero most of the time (which is of course app specific matter, though). Thus, we can avoid lots of overhead.
One more comment on dispatch semaphore, which is the opposite scenario to above. If your threads have different execution priorities, and the higher priority threads have to lock the semaphore for a long time, dispatch semaphore may not be the solution. This is because there's no "queue" among waiting threads. What happens at this case is that higher priority
threads get and lock the semaphore most of the time, and lower priority threads can lock the semaphore only occasionally, thus, mostly just waiting. If this behavior is not good for your application, you have to consider dispatch queue instead.
You can use NSLock or NSRecursiveLock. If you need to call one locking function from another locking function use recursive version.
class X {
let lock = NSLock()
func doSome() {
lock.lock()
defer { lock.unlock() }
//do something here
}
}