Re: [boost] Executor associated to the future continuations (was [thread] boost::future::then)

Subject: Re: [boost] Executor associated to the future continuations (was [thread] boost::future::then)
From: Vicente J. Botet Escriba (vicente.botet_at_[hidden])
Date: 2015-11-06 12:04:22

Le 06/11/15 08:35, Gavin Lambert a écrit :
> On 29/10/2015 23:01, Vicente J. Botet Escriba wrote:
>> Le 29/10/15 10:14, Vladimir Prus a écrit :
>>> It would seem, really, that if an executor was specified in promise,
>>> then using that executor should be default?
>> Well, this is now the default behavior as it corresponds to what was
>> documented.
>>
>> However I believe that the default behavior shouldn't be to inherit.
>> I want to be able to execute the continuation synchronously by the
>> thread that made ready the promise.
>> I have added an experimental flag BOOST_THREAD_CONTINUATION_SYNC that
>> switch the default behavior.
>> Alternatively I could add a launch::sync policy  (and I will do it).
>> So the single thing that will remain is to decide what is the default
>> behavior.
>
> Admittedly I'm not very familiar with the Concurrency TS, but I think 
> the reason that continuations are supposed to be executed via the 
> inherited executor by default is for the 
> marshal-back-to-main/UI-thread case when handing off background work.
>
> In the case where background work is itself registering further 
> continuations, then the background thread is supposed to be running a 
> more generalised executor which can allow synchronous execution (eg. 
> the executor's policy says "must run on one of these threadpool 
> threads", and you happen to already be running on one of those 
> threads, or "can run on any thread").
>
> This is similar behaviour to ASIO's io_service::dispatch.
>
> I don't think trying to decide this at the time the promise is made 
> ready is the right choice.  Unless I'm just misunderstanding what 
> you're referring to here.
>
You misunderstood it. The launch::sync policy is usable with future::then()

f.then launch::sync, next);

Here next continuation will be executed on the same thread that will 
make ready the future.

Vicente