Need advice about adding blocks to a queue (objective-c) using GCD please?

1) No, the global queues have a special role in this. They may execute multiple blocks concurrently on different threads. They are more or less there to have a quick way of entering background jobs if you don't care about ordering (see 1 Blocks submitted to these global concurrent queues may be executed concurrently with respect to each other.

). If you need sequential ordering, create a queue for your task. They are quite lightweight and thus not a big performance impact.

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