When to use Semaphore instead of Dispatch Group?

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.

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))

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.