Re: [boost] ASIO into the standard (was: Re: C++ committee meeting report)

Subject: Re: [boost] ASIO into the standard (was: Re: C++ committee meeting report)
From: Niall Douglas (s_sourceforge_at_[hidden])
Date: 2014-07-04 07:44:45

On 4 Jul 2014 at 11:46, Neil Groves wrote:

> >> I would hope Allocators would be added to ASIO in the standard. It
> >> is difficult to limit memory usage without Allocators in embedded
> >> systems.
> > 
> > I hate this recurring theme of "let's take something that works well
> > today and then fsck it up by insisting it has allocator support"
> 
> Since I am far less familiar with these issues than you are the solution
> to avoiding detriment to designs by having allocator support is not
> obvious to me. What should we do instead of adding allocator support?
> Should we be improving the standard allocator like the implementations
> in Boost.Container, or are you suggesting that any standardisation of
> the allocator Concept would lead to a deterioration of the design? My
> own view is that the standard allocator is clearly suboptimal even for
> standard containers. This is evident from the performance improvements
> obtained in Boost.Container. I have noticed though that the improved
> allocators do not provide superior performance on Linux systems over the
> standard implementations. ITMm very interested in your proposed solutions
> as I suspect there is much to learn from your experience working on the
> standard library implementations. 

There are lots of things going on here. STL allocators were always 
half baked because they were never finished - John Lakos is but one 
of many who took them to all sorts of interesting new places, indeed 
Stepanov's original design had enormous potential. Unfortunately, 
we've ended up stuck with these half finished things, and now they 
are creating slow path performance with std::vector<std::vector<T>> 
because STL containers can't have a noexcept move constructor due to 
allocators!

Allocators are getting patched for C++17 right now, but ultimately 
the problem remains the same one as why they were never finished and 
left half baked - to take them further in any direction will benefit 
some uses but penalise others, so we've collectively decided that's 
as good as it gets for now and left it alone: no one is seriously 
proposing replacing STL allocators completely with something totally 
different.

Regarding ASIO and allocators, ideally ASIO doesn't allocate nor 
deallocate any memory at all during its normal course of execution, 
thus removing much need for allocators and keeping latency low. 
Achieving that is probably hard - just look at promise/future, in 
there is an implied unavoidable malloc/free. And allocator support 
for promise/future really is a mess, indeed Boost.Thread still 
doesn't support that, and if I have my way it never will because 
promise/future under C++11 or later is going to become malloc-free, 
thereby sidestepping the problem.

Niall

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