Concurrent queue calls inside serial queue?

Question

In Objective-C and Swift, is there any guarantee of order of execution for concurrent calls being made inside of a serial queue's async block?

Pseudo-code:

let serialQueue = SerialQueue()
let concurrentQueue = ConcurrentQueue()

serialQueue.async { // 1
    concurrentQueue.async { // 2
        run_task1() // task that takes 1 hour
    }
}

serialQueue.async { // 3
    concurrentQueue.async { // 4
        run_task2() // task that takes 1 minute
    }
}

In the above code, is task1 guaranteed to complete before task2 is called?

Or since they're called on a concurrent thread, the serial queue async only guarantees that run_task1 will be added to the concurrentQueue before run_task2, but not guarantee order of execution?

Alexander · Accepted Answer · 2021-04-22 19:40:50Z

4

I've numbered the block in your question, so I can reference them here:

Block 1 and 3 are both running on a serial queue, thus block 3 will only run once 1 is done.

However, block 1 and 3 don't actually wait for task1/2, they just queue off work to happen asynchronously in blocks 2 and 4, which finishes near instantly.

From then on, both task 1 and 2 will be running concurrently, and finish in an arbitrary order. The only guarantee is that task1 will start before task2.

I always like to use the analogy of ordering a pizza vs making a pizza. Queuing async work is like ordering a pizza. It doesn't mean you have a pizza ready immediately, and you're not going to be blocked from doing other things while the pizzeria is baking your pizza.

Your blocks 1 and 3 are strongly ordered, so 1 will finish and finish before 3 starts. However, all the block does is order a pizza, and that's fast. It does mean pizza 1 (task 1) is done before pizza 2 (task 2), it just means you got off the first phone call before making the second.

edited Apr 22, 2021 at 19:40

answered Apr 22, 2021 at 19:35

Alexander

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7 Comments

dinosaysrawr Over a year ago

cool, so my second assumption at the end of my question is correct, right? block 3 won't wait for block 2 to be finished, block 3 will be called as soon as block 2 is added to the stack, which will be nearly instantaneous

Alexander Over a year ago

@dinosaysrawr it depends what you mean by "order of execution?" exactly. 1 will start before 2, though that's rarely observable. One case is when your system starts running out of threads. Task 1 will start, get the last thread, and task 2 will be forced to wait. The ordering there is definite.

Alexander Over a year ago

"block 3 won't wait for block 2 to be finished" No, althought that's different from what your question says at the end

dinosaysrawr Over a year ago

in that last part, order of execution means task1 completes before task2 begins, which isn't guaranteed and likely won't happen (I know that's not the correct definition of order of execution though haha)

Alexander Over a year ago

Let's avoid the term "execution" and focus specifically on "start" and "end" of each work item. Task 2 certainly can start before task 1 is ends. What's what it means to be a concurrent queue.

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