Let's Build dispatch_queue | Hacker News Reader

Let's Build dispatch_queue

mikeash.com

71 pointsingve23 comments

One of the occasionally annoying things about developer communities is their tendency to present certain technologies as unqualified 'magic' and one should be thankful for developing for platform x because only it has technology y. GCD is sometimes presented this way, when it's mostly just a thread pool with a nice API.

Which is why articles like this are so great. When I was first dipping my toes into development, Mike Ash's articles were a great help in cutting through the marketing hype and realising that these are mostly things you could have implemented yourself given the time and inclination.

While I quite agree with your general point I think GCD is often under appreciated. Characterizing GCD as "mostly just a thread pool with a nice API" glosses over the key feature which Mike Ash specifically points out is not in this implementation: "the number of threads in the global pool scales up and down with the amount of work to be done and the CPU utilization of the system." Making the pool aware of the utilization of the whole system as a whole is the key feature of GCD. This also cuts to why something like this is best if it is created and promoted by the system vendor: the more GCD is utilized by the system software and third party applications the better the system will perform as a whole.

You clearly didn't read the article, which repeatedly states that GCD is not just a thread pool with a nice API.

The whole point of it is that GCD is a lot more efficient than that because it is optimized across the whole system. The author intentionally chooses to omit this because he is trying to present a simplified example for educational purposes.

            if([_blocks count] < idleThreads && _threadCount < _threadCountLimit) {

Shouldn't that be > idleThreads?

Yes indeed! Oddly in the code on GitHub it's correct. I'm not sure how the mismatch happened. I corrected it in the article as well. Thanks for pointing that out.

Perhaps a problem with substituting the correct HTML entities :)

I used to have a lot of problems with that, due to my crappy home-grown blog software, but it's pretty solid now. I think what may have happened is I had it backwards in the code originally, noticed the problem when pasting the code into the article, then fixed the code but forgot to fix the article. Maybe. It was late and beer was involved.

Nice! dispatch is a lovely API. Concurrent queues are nice to have, but the real magic is in serial queues.

For example, we can wrap a hash table in a dispatch queue: get() uses dispatch_sync, set() uses dispatch_async, and boom: our hash table rehashes in the background, instead of forcing set() to wait.

What are some other APIs that enable you to build a hash table like that?

For others not knowing the details of Cocoa API and wondering about it, [NSCondition wait] unlocks the condition's locks and blocks the thread until the condition variable is signaled, at which point is reacquires the lock and resumes.

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