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

[Wallacy]

Hello,

I have a little question about the actors.

On WWDC 2012 Session 712 one of the most important tips (for me at least)
was: Improve Performance with Reader-Writer Access

Basically:
• Use concurrent subsystem queue: DISPATCH_QUEUE_CONCURRENT
• Use synchronous concurrent “reads”: dispatch_sync()
• Use asynchronous serialized “writes”: dispatch_barrier_async()

Example:

// ...
   _someManagerQueue = dispatch_queue_create("SomeManager",
DISPATCH_QUEUE_CONCURRENT);// ...


And then:

- (id) getSomeArrayItem:(NSUInteger) index {
    id importantObj = NULL;
    dispatch_sync(_someManagerQueue,^{
        id importantObj = [_importantArray objectAtIndex:index];
     });
   return importantObj;
 }- (void) removeSomeArrayItem:(id) object {
     dispatch_barrier_async(_someManagerQueue,^{
         [_importantArray removeObject:object];
     });
 }- (void) addSomeArrayItem:(id) object {
     dispatch_barrier_async(_someManagerQueue,^{
         [_importantArray addObject:object];
     });
 }


That way you ensure that whenever you read an information (eg an array) all
the "changes" have been made ​​or are "waiting" . And every time you write
an information, your program will not be blocked waiting for the operation
to be completed.

That way, if you use several threads, none will have to wait another to get
any value unless one of them is "writing", which is right thing to do.

With this will it be composed using actors? I see a lot of discussion about
using serial queues, and I also have not seen any mechanism similar to
dispatch_barrier_async being discussed here or in other threads.

Em seg, 4 de set de 2017 às 08:20, Daniel Vollmer via swift-evolution <
swift-evolution at swift.org> escreveu:

> Hello,
>
> first off, I’m following this discussion with great interest, even though
> my background (simulation software on HPC) has a different focus than the
> “usual” paradigms Swift seeks to (primarily) address.
>
> > On 3. Sep 2017, at 19:26, Chris Lattner via swift-evolution <
> swift-evolution at swift.org> wrote:
> >> On Sep 2, 2017, at 11:09 PM, Pierre Habouzit <phabouzit at apple.com>
> wrote:
> >>> On Sep 2, 2017, at 12:19 PM, Pierre Habouzit <pierre at habouzit.net>
> wrote:
> >>>
> >>> Is there a specific important use case for being able to target an
> actor to an existing queue?  Are you looking for advanced patterns where
> multiple actors (each providing disjoint mutable state) share an underlying
> queue? Would this be for performance reasons, for compatibility with
> existing code, or something else?
> >>
> >> Mostly for interaction with current designs where being on a given
> bottom serial queue gives you the locking context for resources naturally
> attached to it.
> >
> > Ok.  I don’t understand the use-case well enough to know how we should
> model this.  For example, is it important for an actor to be able to change
> its queue dynamically as it goes (something that sounds really scary to me)
> or can the “queue to use” be specified at actor initialization time?
>
> I’m confused, but that may just be me misunderstanding things again. I’d
> assume each actor has its own (serial) queue that is used to serialize its
> messages, so the queue above refers to the queue used to actually process
> the messages the actor receives, correct?
>
> Sometimes, I’d probably make sense (or even be required to fix this to a
> certain queue (in the thread(-pool?) sense), but at others it may just make
> sense to execute the messages in-place by the sender if they don’t block so
> no context switch is incurred.
>
> > One plausible way to model this is to say that it is a “multithreaded
> actor” of some sort, where the innards of the actor allow arbitrary number
> of client threads to call into it concurrently.  The onus would be on the
> implementor of the NIC or database to implement the proper synchronization
> on the mutable state within the actor.
> >>
> >> I think what you said made sense.
> >
> > Ok, I captured this in yet-another speculative section:
> >
> https://gist.github.com/lattner/31ed37682ef1576b16bca1432ea9f782#intra-actor-concurrency
>
> This seems like an interesting extension (where the actor-internal serial
> queue is not used / bypassed).
>
>
>         Daniel.
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>
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