How to synchronize tasks in different dispatch queues?

Question

I'm new to queues and I'm having some trouble setting up the following scheme.

I have three tasks that need doing.

Task A: Can only run on the main queue, can run asynchronously with task B, cannot run asynchronously with task C. Runs a lot but runs fairly quickly.

Task B: Can run on any queue, can run asynchronously with task A, cannot run asynchronously with task C. Runs rarely, but takes a long time to run. Needs Task C to run afterwards, but once again task C cannot run asynchronously with task A.

Task C: Can run on any queue. Cannot run asynchronously with either task A or task B. Runs rarely and runs quickly.

Right now I have it like this:

Task A is submitted to the main queue by a Serial Queue X (a task is submitted to Serial Queue X to submit task A to the main queue).

Task B is submitted to Serial Queue X.

Task C is submitted to the main queue by Serial Queue X, just like task A.

The problem here is that task C sometimes runs at the same time as task B. The main queue sometimes runs task C at the same time that the serial queue runs task B.

So, how can I ensure that task B and task C never run at the same time while still allowing A and B to run at the same time and preventing A and C from running at the same time? Further, is there any easy way to make sure they run the same number of times? (alternating back and forth)

Answer 1

You know, I think I had this problem on my GRE, only A, B, and C were Bob, Larry, and Sue and they all worked at the same office.

I believe that this can be solved with a combination of a serial queue and a dispatch semaphore. If you set up a single-wide serial dispatch queue and submit tasks B and C to that, you'll guarantee that they won't run at the same time. You can then use a dispatch semaphore with a count set to 1 that is shared between tasks A and C to guarantee that only one of them will run at a time. I describe how such a semaphore works in my answer here. You might need to alter that code to use DISPATCH_TIME_FOREVER so that task A is held up before submission rather than just tossed aside if C is running (likewise for submission of C).

This way, A and B will be running on different queues (the main queue and your serial queue) so they can execute in parallel, but B and C cannot run at the same time due to their shared queue, nor can A and C because of the semaphore.

As far as load balancing on A and C goes (what I assume you want to balance), that's probably going to be fairly application-specific, and might require some experimentation on your part to see how to interleave actions properly without wasting cycles. I'd also make sure that you really need them to alternate evenly, or if you can get by with one running slightly more than another.