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Subject: Re: [boost] Futures (was: Re: [compute] Some remarks)
From: Niall Douglas (s_sourceforge_at_[hidden])
Date: 2015-01-05 06:01:53

On 4 Jan 2015 at 9:52, Hartmut Kaiser wrote:

> > 1. They tie your code into "future islands" which are fundamentally
> > incommensurate with all code which doesn't use the same future as
> > your code. Try mixing code using boost::future and std::future for
> > example, it's a nightmare of too easy to be racy and unmaintainable
> > mess code. If Compute provided a boost::compute::future, it would yet
> > another new future island, and I'm not sure that's wise design.
> The easiest way to deal with this is to introduce a Future concept and
> implement everything in terms of it. A solid set of traits/concepts-lite
> should cover that.

I don't think it's that easy because really it comes down to
commonality of kernel wait object, or rather, whether one has access
to the true underlying kernel wait object or not.

For example, right now can boost::wait_all() ever consume
std::futures? I suspect not because the HANDLE on Windows or the
futex on Linux is rather hard to get at.

> A good example is the proposed await 2.0 (see N4134:
>, btw. The
> proposed await keyword can be adapted to handle arbitrary future types using
> a user supplied trait. We tried that with hpx::future and it seems to work
> fine (once they fix a compiler bug which prevented us from doing large scale
> tests).

N4134 presents excellent progress, apart from one major thing I
disliked about it: I deeply dislike magic compiler tricks which fold
the allocation of shared state by a future promise onto the stack.

Far, far better to fix the present future promise API to stop
requiring shared state and therefore memory allocation at all.

> > 2. Every time you touch them with change you unavoidably spend
> > thousands of CPU cycles due to going through the memory allocator and
> > (effectively) the internal shared_ptr. This makes using futures for a
> > single SHA round, for example, a poor design despite how nice and
> > clean it is.
> As long as the overheads of managing the future itself are much smaller than
> the overheads introduced by the underlying threading system we're fine. And
> for std::future and boost::future this is definitely the case as both are
> tied to kernel-threads. In HPX this is a bigger problem as the overheads
> introduced by futures are comparable with those of the underlying threading
> system (sub-microsecond). However in our experience this is solvable (things
> like special allocators for the shared state and using intrusive_ptr for it
> come to mind).

I think a one size fits all future is a fundamentally flawed

> > 3. They force you to deal with exceptions even where that is not
> > appropriate, and internally most implementations will do one or more
> > internal throw-catches which if the exception type has a vtable, can
> > be particularly slow.
> The implementations will throw only if there is an error. This is a no-issue
> for the non-exceptional case. And I personally don't care if the exceptional
> case is slow (involves things like logging anyways, etc.).

That's not the problem. The problem is that the compiler cannot know
if no exception will ever be generated and therefore has to generate
the opcodes anyway. What I'm really asking for is a "noexcept future"
such that this sequence:

  promise<int> p;
  auto f(p.get_future());
  return f.get();

... can be optimised by the compiler into:

_Z5test1v: # @_Z5test1v
# BB#0: # %_ZN7promiseIiJEED2Ev.exit2
        movl $5, %eax

Obviously this is an unrealistic use case, but my point is that the
compiler should be capable of such a reduction because the correct
design of future promise wouldn't get in the way.

My earlier proposal for non-allocating future promise doesn't even
transport a value it's so basic (hence "basic_future" and
"basic_promise"). It pushes that responsibility onto something like
expected<T, E> where if the programmer chooses, E can be a
std::exception_ptr in which case you get exception capable futures.
Or or E can be a std::error_code, in which case the future is now
noexcept and the compiler can completely elide all related exception
handling machinery.

Obviously one could use an expected<expected<T, error_code>,
exception_ptr> too. In fact, this is exactly what I need for AFIO and
is why I started down this rabbit hole at all. Personally speaking, I
have better things to be doing than working on futures, but right now
they are a major showstopper for me, particularly for efficient SHA

> > Replacing the entire concurrency engine and indeed paradigm in your
> > C++ runtime is, I suspect, too scary for most, even if the code
> > changes are straightforward. It'll be the "bigness" of the concept
> > which scares them off.
> What if the whole std library was based on something like HPX? In this case
> the user wouldn't have to care about this anymore, right?

That works for me. I just don't want yet another STL implementation
to support.

> > To that end, the non-allocating basic_future toolkit I proposed on
> > this list before Christmas I think has the best chance of "fixing"
> > futures. Each programmer can roll their own future type, with
> > optional amounts of interoperability and composure with other future
> > islands. Then a future type lightweight enough for a SHA round is
> > possible, as is some big thick future type providing STL future
> > semantics or composure with many other custom future types. One also
> > gains most of the (static) benefits of ASIO's async_result, but one
> > still has ABI stability.
> Non-allocating futures are a step in the right direction. But even those
> require to solve some of the problems you mentioned. Otherwise they will
> make the issue of having future-islands just a bit bigger...

Eliminating future islands is, I suspect, not something the C++
community can entirely do alone. We are, as a minimum, going to have
to petition POSIX for improved runtime support. We probably ought to
have our ducks in a row before that though.


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