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Subject: Re: [boost] Boost.Fiber review January 6-15
From: Antony Polukhin (antoshkka_at_[hidden])
Date: 2014-01-10 05:02:23

2014/1/10 Oliver Kowalke <oliver.kowalke_at_[hidden]>

> 2014/1/10 Antony Polukhin <antoshkka_at_[hidden]>
> > Scheduler does not look good. Class "algorithm" looks overcomplicated:
> too
> > many methods, classes from detail namespace are used in function
> > signatures, looks like some methods duplicate each other (yield, wait).
> >
> which one should be removed an why?

Do not remove schedulers. Just remove "algorithm" from docs and hide it in
namespace detail.

> algorithm::wait() and algorithm::yield() have different effect on the fiber
> algorithm::wait() sets the internal state of a fiber to WAITING suspends
> and stores it
> into an internal queue (waiting-queue). the fiber must be signaled to be
> ready in order to
> moved from the waiting- to the ready-queue so it can be resumed later.
> in contrast to this algorithm::yield() suspends the fiber, let the internal
> state be READY and appends it at the end of
> the ready-queue so that it does not have to be signaled etc.

Oh, now I see.

> > I'm slightly confused by the fact that fiber_base use many dynamic memory
> > allocations (it contains std::vector, std::map) and virtual functions.
> > There must be some way to optimize away that, otherwise fibers may be
> much
> > slower than threads.
> >
> hmm - boost.thread uses stl containers too (for instance thread_data_base)
> the fiber must hold some data by itself, for instance a list of fibers
> waiting on it to terminated (join).
> otherwise thread migration between thread would be hard to implement if not
> impossible.

My concern is that fiber migration between threads is a rare case. So first
of all fibers must be optimized for a single threaded usage. This means
that all the thread migration mechanics must be put in scheduler, leaving
fiber light and thread unsafe.

What if round_robin scheduler be a thread local variable (just like now,
but without any thread sync), but for cases when fiber migration is
required a global round_robin_ws scheduler is created for all the threads?
In that way we'll get high performance in single threaded applications and
hide all the thread migrations and sync inside the thread safe
round_robin_ws scheduler.

> > Back to the fiber_base. There's too many atomics and locks in it. As I
> > understand, the default use-case does not include thread migrations so
> > there is no need to put all the synchronizations inside the fiber_base.
> > round_robin_ws and mutexes work with multithreading, ideologically it's
> the
> > more correct place for synchronizations. Let fiber_base be thread unsafe,
> > and schedulers and mutexes take care about the serializing access to the
> > fiber_base. Otherwise singlethreaded users will loose performance.
> >
> the only way to do this would be that every fiber_base holds a pointer to
> its fiber-scheduler (algorithm *).
> in the case of fibers joining the current fiber (e.g. stored in the
> internal list of fiber_base) the fiber
> has to signal the schduler of the joining fiber.
> BOOST_FOREACH( fiber_base::ptr_t f, joining_fibers) {
> f->set_ready() // fiber_base::set_ready() { scheduler->set_ready(
> this); }
> }
> this pattern must be applied to mutex and condition_variable too.
> if a fier is migrated its point to the scheduler must be swapped.

Is that easy to implement? Does it scale well with proposal of thread
unsafe round_robin and thread safe round_robin_ws?

Best regards,
Antony Polukhin

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