Re: [boost] [c++std-parallel-2020] Re: We need a coherent higher level parallelization story for C++ (was [thread] Is there a non-blocking future-destructor?)

Subject: Re: [boost] [c++std-parallel-2020] Re: We need a coherent higher level parallelization story for C++ (was [thread] Is there a non-blocking future-destructor?)
From: Peter Dimov (pdimov_at_[hidden])
Date: 2015-10-14 12:47:51

Lawrence Crowl wrote:

> First, std::async and std::future have no flaws for what they were 
> designed to do.  They were designed to provide relatively easy parallelism 
> for small core systems under the fork-join model.

I thought that it was well established by now that std::async isn't very 
useful for fork-join (if at all.)

First, it's required to execute each (asynchronous) task as-if in a new 
thread with respect to thread locals, which in practice means "in a new 
thread".

Second, it has to allocate the future shared state.

These two non-flaws kill its performance for fork-join parallelism.

The first one is not related to its use of std::future, but the second one 
is. Given an interface of the form

    X x;
    async_result<R> r;

    std::async( r, [&]{ return x.f(); } );

with async_result being non-copyable and non-movable and ~async_result 
blocking on the async task, the task can store its result directly into r, 
without needing to allocate.