How does DispatchQueue.global() differ from your own concurrent queue?

Middle PRO

The essence

DispatchQueue.global(qos:) is one of the system concurrent queues (one per QoS level), shared by the whole process: Apple frameworks and third-party libraries put their work there too. Your own DispatchQueue(label:attributes: .concurrent) is a private object that only you control.

Practical differences:

• Barrier. .barrier only makes sense on your own concurrent queue. On a global queue the flag is ignored (otherwise one barrier would stall the whole system) — the block runs as a plain async.
• Isolation. Only your tasks run on your queue, so sync/barrier give predictable guarantees and it can serve as a reader-writer lock.
• Control. Your own queue can be suspended (suspend/resume), given a target queue, an autoreleaseFrequency, a default QoS; a global one cannot.
• Debugging. The label of your queue shows up in Instruments and crash stacks.
• Resources. By default your queue targets the global queue of the same QoS, so threads come from the same pool — it creates no new threads.