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Subject: [boost] Futures (was: Re: [compute] Some remarks)
From: Niall Douglas (s_sourceforge_at_[hidden])
Date: 2015-01-04 01:52:41

On 3 Jan 2015 at 7:15, Hartmut Kaiser wrote:

> First of all, I fully support Thomas here. Futures (and the extensions
> proposed in the 'Concurrency TS') are a wonderful concept allowing
> asynchronous computation. Those go beyond 'classical' futures, which just
> represent a result which has not computed yet. These futures allow for
> continuation style coding as you can attach continuations and compose new
> futures based on logical operations on others.

They are also severely limited and limiting:

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.

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.

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.

4. The compiler's optimiser really struggles to do much with the
current future design because of all the implicit visibility to other
threads. Even a very simple use of future requires hundreds of CPU
instructions to be generated as a minimum, none of which can be
elided because the compiler can't know visibility effects to other
threads. I'll grant you that a HPX type design makes this problem
much more tractable because the real problem here is the potential
presence of hardware concurrency.

This is why Chris has proposed async_result from ASIO instead, that
lets the caller of an async API supply the synchronisation method to
be used for that particular call. async_result is superior to futures
in all but one extremely important way: async_result cannot traverse
an ABI boundary, while futures can.

> What do you mean by 'making everything a future'? Having all functions
> return futures? If so - then yes - if you want to make a function
> asynchronously callable, let it return a future. There is nothing wrong with
> that (well, except that std::future is utterly bulky and slow as it is
> usually tied to std::sthread which in turn is usually representing kernel
> threads - for a proposed solution see my talk at MeetingC++ 2014 [2]).

For the record, I'd just love if there were more HPX type thinking in
how C++ concurrency is standardised.

However, I have learned with age and experience that people don't
care much for whole new ways of thinking and approaching problems.
They prefer some small incremental library which can be tacked onto
their existing code without much conceptual change. To that end, when
facing the limitations of std::future they can see the cost-benefit
of boost:future, and can conceptualise replacing std::future with
boost::future in their code. So that is a viable mental step for

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.

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.


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