There is an interesting way to create a GCD queue that can be both concurrent and serial, to say allow thread safe "reads" from a structure to occur concurrently in any order, but only permit "write" to occur synchronously after all queued reads have completed. For example to access items where reads may actually take some time to search the structure. But writes must occur serially to prevent altering the structure while a read is taking place. Refer to the Related item on "barrier" for how to do this.