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Subject: Re: [boost] [next gen future-promise] What to call the monadic return type?
From: Emil Dotchevski (emildotchevski_at_[hidden])
Date: 2015-05-26 21:01:31

On Tue, May 26, 2015 at 5:29 PM, Hartmut Kaiser <hartmut.kaiser_at_[hidden]>

> > > It can be important for O_DIRECT AIO operations. I agree that for
> > > buffered I/O, the filesystem overhead will dominate (and, on Linux, you
> > > don't have a way to implement futures over buffered I/O without
> > > resorting to threads, which will slow things down further).
> >
> > Actually, on recent Linuces with ext4 reads from page cached data are
> > now kaio wait free. It makes a big difference for warm cache
> > filesystem when you're doing lots of small reads. Writes
> > unfortunately still lock, and moreover exclude readers. Linux has a
> > very long way to go to reach BSD and especially Windows for async
> > i/o.
> Optimizing away one allocation to create a promise/future pair (or just a
> future for make_ready_future) will have no measurable impact in the context
> of any I/O, be it wait free or asynchronous or both.
> In general, all I'm hearing on this thread is 'it could be helpful', 'it
> should be faster', 'it can be important', or 'makes a big difference', etc.
> I was hoping that we as a Boost community can do better!
> Nobody so far has shown the impact of this optimization technique on a real
> world applications (measurements). Or at least, measurement results from
> artificial benchmarks under heavy concurrency conditions (using decent
> multi-threaded allocators like jemalloc or tcmalloc). I'd venture to say
> that there will be no measurable speedup (unless proven otherwise).


I'd add that not only there should be a measurable improvement, but it
should be a measurable improvement compared to other reasonable
optimizations, for example allocations can be optimized through custom
allocators. As an analogy, it's not sufficient to show that shared_ptr is
"too slow" or "allocates too much" compared to some other smart pointer
type -- it also must be shown that the slowness can't be trivially dealt
with by implementing a custom allocator for some shared_ptr instances, if

Emil Dotchevski
Reverge Studios, Inc.

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