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Subject: [boost] Universal async in C++ (was: Re: [Fibers] Performance)
From: Niall Douglas (s_sourceforge_at_[hidden])
Date: 2014-01-21 13:20:27

On 21 Jan 2014 at 10:09, Nat Goodspeed wrote:

> >>
> > That's a good paper, but I wish it didn't claim to be a *universal*
> > model for asynchronous operations because that model is completely
> > unsuitable for persistent storage i/o. I had that argument with
> > Nicholas @ Microsoft Research actually, and I think I may well have
> > persuaded him as they're seeing the same problems of fit.
> Niall, would you be able to propose a more universal model? Please
> read this as a simple invitation rather than a challenge. The goal of
> the paper seems laudable: accepting an argument that allows the caller
> to specify whether to provide results with a callback, a future or
> suspend-and-resume.

I think I effectively have via AFIO which uses an asynchronous
execution precedence graph model with explicit gathering for error
propagation, which unlike the proposed model works as well with
seekable i/o as fifo i/o. The problem with the AFIO model though is
that it is very heavily reliant on the performance of futures (and
therefore the memory allocator) as those are used to transport state
between closures, and for those with very low latency very small
packet (e.g. UDP) socket i/o it's unsuitable (which is of course
pointed out in the WG21 paper). My opinion there is that callbacks
can become a low level interface for those who need it, while the
execution precedence graph model is much easier to program against
for everything else.

You're probably about to ask me for a N-paper, so I'll save an email
cycle by explaining my hesitation on that. Bjorn Reece has been
working extensively with me off list to de-wart AFIO when combined
with ASIO such that async socket and async disc i/o seamlessly
interoperate, specifically he's persuaded me to supply async_io_op to
completion handlers which is going to be a technical challenge for me
to implement in a safe and quick way, but I think I am capable though
it's going to hurt. I will start that work item once I clear my maths
coursework (hopefully tomorrow), and file my consulting company's
annual accounts (hopefully end of this week). Also, I might actually
have a job interview soon, which amazes me as I had expected at least
three months to elapse before any jobs in C++ turned up (there are
about six a year total in this region).

Both Artur and Niklas who are leading out Microsoft's work on async
in C++ are aware of AFIO's design approach, and last time I heard
they found themselves coming ever closer to AFIO's design as they
find blocking issues and corner cases in their proposals. I think it
will be more productive for now for me to keep nagging them on their
N-papers rather than write one of my own. After all, being in Europe
and being unemployed makes it extremely tough to attend C++ standards
meetings, and I don't personally rate a N paper's chances if someone
isn't at the meetings to champion it i.e. present in the bar
afterwards to argue merits with people.

Hopefully this explains things. Do point out any problems in my


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