[swift-evolution] [Concurrency] async/await + actors

Charles Srstka cocoadev at charlessoft.com
Sat Sep 2 16:19:15 CDT 2017

> On Sep 2, 2017, at 4:05 PM, David Zarzycki via swift-evolution <swift-evolution at swift.org> wrote:
>> On Sep 2, 2017, at 14:15, Chris Lattner via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> My understanding is that GCD doesn’t currently scale to 1M concurrent queues / tasks.
> Hi Chris!
> [As a preface, I’ve only read a few of these concurrency related emails on swift-evolution, so please forgive me if I missed something.]
> When it comes to GCD scalability, the short answer is that millions of of tiny heap allocations are cheap, be they queues or closures. And GCD has fairly linear performance so long as the millions of closures/queues are non-blocking.
> The real world is far messier though. In practice, real world code blocks all of the time. In the case of GCD tasks, this is often tolerable for most apps, because their CPU usage is bursty and any accidental “thread explosion” that is created is super temporary. That being said, programs that create thousands of queues/closures that block on I/O will naturally get thousands of threads. GCD is efficient but not magic.
> As an aside, there are things that future versions of GCD could do to minimize the “thread explosion” problem. For example, if GCD interposed the system call layer, it would gain visibility into *why* threads are stalled and therefore GCD could 1) be more conservative about when to fire up more worker threads and 2) defer resuming threads that are at “safe” stopping points if all of the CPUs are busy.
> That being done though, the complaining would just shift. Instead of an “explosion of threads”, people would complain about an “explosion of stacks" that consume memory and address space. While I and others have argued in the past that solving this means that frameworks must embrace callback API design patterns, I personally am no longer of this opinion. As I see it, I don’t think the complexity (and bugs) of heavy async/callback/coroutine designs are worth the memory savings. Said differently, the stack is simple and efficient. Why fight it?
> I think the real problem is that programmers cannot pretend that resources are infinite. For example, if one implements a photo library browsing app, it would be naive to try and load every image at launch (async or otherwise). That just won’t scale and that isn’t the operating system's fault.

Problems like thread explosion can be solved using higher-level constructs, though. For example, (NS)OperationQueue has a .maxConcurrentOperationCount property. If you make a global OperationQueue, set the maximum to whatever you want it to be, and run all your “primitive” operations through the queue, you can manage the thread count rather effectively.

I have a few custom Operation subclasses that easily wrap arbitrary asynchronous operations as Operation objects; once the new async/await API comes out, I plan to adapt my subclass to support it, and I’d be happy to submit the code to swift-evolution if people are interested.


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