[libdispatch-dev] Queue size

Dave Zarzycki zarzycki at apple.com
Thu Oct 7 08:14:04 PDT 2010

On Oct 7, 2010, at 10:57 AM, Thomas Clement wrote:

> On Oct 7, 2010, at 3:10 PM, Dave Zarzycki wrote:
>> On Oct 7, 2010, at 8:35 AM, Thomas Clement wrote:
>>>> Queues cannot have variable widths - it's either 1 or "infinite".  That idea was considered but ultimately dropped given that it only made submitting things to queues even less deterministic in terms of where/how they would run, and there is already a way (again, semaphores) to get that behavior, so this would have only added complexity to all queues for little overall gain.
>>> I'm confused. What about the dispatch_queue_set_width() private function?
>>> Isn't this already implemented and functional?
>> "Implemented and functional" is not the bar we use for making an API publicly available. It also has to be sustainable and supportable.
>> Unfortunately, dispatch_queue_set_width() fails at the latter goals. That API encourages bad design, and practically speaking, it was only created so that developers can workaround underlying bugs (latent serialization). We'd much rather see developers fix (or file bugs) against the underlying problems than see a hierarchy of long term bandaids and workarounds be created.
> I understand.
> The problem I'm facing is that some of my dispatched code at some point pauses its execution (basically locking on pthread_cond_wait() or similar) and libdispatch spawns way too many threads as a result (hundreds of them).


That would be a "latent serialization" problem that was hinted at in my response. The dirty secret of our industry is that many thread safe libraries are not actually designed to achieve any concurrency when invoked concurrently. This creates huge problems for developers like yourself that are trying to use them concurrently.

> It is difficult if not impossible to remove these locks. I guess the solution is to limit the number of dispatched blocks on my queues using dispatch semaphores.

The solution is to desynchronize the subsystem in question and switch to completion callbacks. In other words, do this kind of transform:

	Result *foo(Input *x);


	void foo(Input *x, dispatch_queue_t completion_queue, void (^completion_callback)(Result *));


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