SerialDispatchQueue why need two queues(externalQueue and localQueue)?

[张伟]

  I read SerialDispatchQueue source code, and I find that there are two task queues in it. Why two queues? And why two queues have difffrent type: ConcurrentLinkedQueue and LinkedList?

[Hiram Chirino]

It's an optimization for when you add more tasks to the dispatch queue
which is currently executing on the current thread. Adding to the
non-thread safe LinkedList is much faster than adding to the
ConcurrentLinkedQueue.

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Hiram Chirino

Engineering | Red Hat, Inc.

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[Christian Posta]

Hiram,

What happens when a serial dispatch queue is executing on a global queue thread but a different global queue thread tries to enqueue to the serial dispatch queue, or some non hawtdispatch thread tries enqueue to that serial dispatch queue... Seems the non thread safe data structure is still used? Or is that "is executing" guard in the enqueue method specific to "is executing on current thread"?

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Christian Posta

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twitter: @christianposta

[Hiram Chirino]

Yes. The "is executing" guard is specific to "is executing on current thread".