serial Dispatch Queue will use only one thread?

Question

I find that serial queue will use more than one thread to run async code. Here's the test code in playground.

import Foundation  

let q = DispatchQueue(label: "test")  
q.async {  
    print("hi \(Thread.current)")  
}  
q.async {  
    print("hi \(Thread.current)")  
}  
q.async {  
    print("hi \(Thread.current)")  
}  
q.async {  
    print("hi \(Thread.current)")  
}  
q.async {  
    print("hi \(Thread.current)")  
}

when I repeatedly execute the playground, there will be output like this sometimes. In my understanding, serial queue should use only one thread, but the log shows it used 2 threads. I am really confused on this. What is the correct behavior?

hi <NSThread: 0x7fc26a467b90>{number = 2, name = (null)}  
hi <NSThread: 0x7fc26a467b90>{number = 2, name = (null)}  
hi <NSThread: 0x7fc26a467b90>{number = 2, name = (null)}  
hi <NSThread: 0x7fc26a467b90>{number = 2, name = (null)}  
hi <NSThread: 0x7fc26b1003e0>{number = 3, name = (null)}

["Serial queues are not guaranteed to perform on the same thread."](https://stackoverflow.com/a/15620610/5228431) — Eduard, Jun 27 '18 at 08:26
In general there is no relationship between queues and threads; the exception being the main queue. Tasks dispatched on the main queue are guaranteed to run on the main thread. Tasks dispatched on any other queue can run on any available thread. A serial dispatch queue ensures that only one task submitted to the queue runs at a time, but guarantees nothing about the thread. — Paulw11, Jun 27 '18 at 09:12

score 6 · Accepted Answer · answered Jan 18 '20 at 17:59

You are right in saying that serial queue will only use 1 thread at "a" time.

But will the same thread (NSThread object) execute each block of code that's enqueued into the queue? No.

To add more clarity to the first statement, I will rephrase it..

Serial queue will only use 1 thread at a time per block execution. Which means that two threads will never act upon any block of code at any point in time.

What thread executes the code that's enqueued/dequeued FIFO order from the queue is not guaranteed. But what is guaranteed is that any "one" arbitrary worker thread will perform the action.

As far as I know, internally, for each block the thread allocation is automatic. "One thread only" does not mean "same thread always".

serial Dispatch Queue will use only one thread?

1 Answers1