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: Gavin Lambert (gavinl_at_[hidden])
Date: 2015-11-06 02:35:34

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.