Does QOS based DispatchSemaphore blocks the entire thread pool

Question

let semaphore = DispatchSemaphore(value: 1) 
DispatchQueue.global(qos: .background)

Does this mean if i use this semaphore to make a specific block of resource to be accessed by only one thread at a time, it will block the entire threads running in the background ( or ) only blocks the threads that demands the resources

Answer 1

It will block only the threads that demand already allocated resources. Whenever a thread calls,

@discardableResult func signal() -> Int 

semaphore count is incremented by 1 and that thread is given access to the critical section. If an upcoming thread calls the same method it will be blocked until the first thread calls

func wait()

method.