> Where do they say that? That's surely wrong. The man page says that in that
> case it returns the default-priority global [serial] queue.
Nevermind, I'm not paying enough attention. I figured you'd have to return a
serial queue, so my brain conveniently ignored the fact that the global default
queues are all concurrent.
FWIW here's the state of affairs on 10.7.2:
Main queue: com.apple.main-thread (0x7fff7c2aa980)
Default concurrent queues:
High priority: com.apple.root.high-priority (0x7fff7c2ab240)
High priority (overcommit): com.apple.root.high-overcommit-priority
(0x7fff7c2ab300)
Default priority: com.apple.root.default-priority (0x7fff7c2ab0c0)
Default priority (overcommit):
com.apple.root.default-overcommit-priority (0x7fff7c2ab180)
Low priority: com.apple.root.low-priority (0x7fff7c2aaf40)
Low priority (overcommit): com.apple.root.low-overcommit-priority
(0x7fff7c2ab000)
Background priority: com.apple.root.background-priority (0x7fff7c2ab3c0)
Current queue as seen by:
Default priority concurrent queue: com.apple.root.default-priority
(0x7fff7c2ab0c0)
Main thread (outside of dispatch queue): com.apple.main-thread
(0x7fff7c2aa980)
Random pthread: com.apple.root.default-overcommit-priority
(0x7fff7c2ab180)
Main queue: com.apple.main-thread (0x7fff7c2aa980)
Note that it returns an over-commit queue if there is no other answer.
Over-commit queues differ from the normal queues in that they have no limit to
the number of threads that may be servicing them concurrently (other than the
general limits on thread numbers). I expect there's a very deliberate reason
why this is done, but it does open the possibility for poorly written code to
over-subscribe the system and bring down the overall performance.
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