EQ2EMu/EQ2/source/depends/boost_1_72_0/libs/fiber/doc/rationale.qbk

[/
          Copyright Oliver Kowalke 2013.
 Distributed under the Boost Software License, Version 1.0.
    (See accompanying file LICENSE_1_0.txt or copy at
          http://www.boost.org/LICENSE_1_0.txt
]

[section:rationale Rationale]

[heading preprocessor defines]

[table preopcessor defines
    [[] []]
    [
        [BOOST_FIBERS_NO_ATOMICS]
        [no `std::atomic<>` used, inter-thread synchronization disabled]
    ]
    [
        [BOOST_FIBERS_SPINLOCK_STD_MUTEX]
        [use `std::mutex` as spinlock instead of default `XCHG`-sinlock with backoff]
    ]
    [
        [BOOST_FIBERS_SPIN_BACKOFF]
        [limit determines when to used `std::this_thread::yield()` instead of
        mnemonic `pause/yield` during busy wait (apllies on to `XCHG`-spinlock)]
    ]
    [
        [BOOST_FIBERS_SINGLE_CORE]
        [allways call `std::this_thread::yield()` without backoff during busy wait
        (apllies on to `XCHG`-spinlock)]
    ]
]


[heading distinction between coroutines and fibers]

The fiber library extends the coroutine library by adding a scheduler and
synchronization mechanisms.

* a coroutine yields
* a fiber blocks

When a coroutine yields, it passes control directly to its caller (or, in the
case of symmetric coroutines, a designated other coroutine). When a fiber
blocks, it implicitly passes control to the fiber scheduler. Coroutines have
no scheduler because they need no scheduler.[footnote
[@http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n4024.pdf 'N4024:
Distinguishing coroutines and fibers']].


[heading what about transactional memory]

GCC supports transactional memory since version 4.7. Unfortunately tests show
that transactional memory is slower (ca. 4x) than spinlocks using atomics.
Once transactional memory is improved (supporting hybrid tm), spinlocks will
be replaced by __transaction_atomic{} statements surrounding the critical
sections.


[heading synchronization between fibers running in different threads]

Synchronization classes from __boost_thread__ block the entire thread. In
contrast, the synchronization classes from __boost_fiber__ block only specific
fibers, so that the scheduler can still keep the thread busy running other
fibers in the meantime.
The synchronization classes from __boost_fiber__ are designed to be
thread-safe, i.e. it is possible to synchronize fibers running in different
threads as well as fibers running in the same thread. (However, there is a
build option to disable cross-thread fiber synchronization support; see [link
cross_thread_sync this description].)

[#spurious_wakeup]
[heading spurious wakeup]

Spurious wakeup can happen when using
[@http://en.cppreference.com/w/cpp/thread/condition_variable
`std::condition_variable`]: the condition variable appears to be have been
signaled while the awaited condition may still be false. Spurious wakeup can
happen repeatedly and is caused on some multiprocessor systems where making
`std::condition_variable` wakeup completely predictable would slow down all
`std::condition_variable` operations.[footnote David R. Butenhof ["Programming
with POSIX Threads]]

__condition__ is not subject to spurious wakeup. Nonetheless it is
prudent to test the business-logic condition in a `wait()` loop [mdash] or,
equivalently, use one of the `wait( lock, predicate )` overloads.

See also [link condition_variable_spurious_wakeups No Spurious Wakeups].


[heading migrating fibers between threads]

Support for migrating fibers between threads has been integrated. The
user-defined scheduler must call __context_detach__ on a fiber-context on the
source thread and __context_attach__ on the destination thread, passing the
fiber-context to migrate. (For more information about custom schedulers, see
[link custom Customization].)
Examples `work_sharing` and `work_stealing` in directory `examples` might be
used as a blueprint.

See also [link migration Migrating fibers between threads].


[heading support for Boost.Asio]

Support for __boost_asio__[s] __async_result__ is not part of the official API.
However, to integrate with a __io_service__, see [link integration Sharing a
Thread with Another Main Loop]. To interface smoothly with an arbitrary Asio
async I/O operation, see [link callbacks_asio Then There[s] __boost_asio__].

[heading tested compilers]

The library was tested with GCC-5.1.1, Clang-3.6.0 and MSVC-14.0 in c++11-mode.


[heading supported architectures]

__boost_fiber__ depends on __boost_context__ - the list of supported architectures
can be found [@http://www.boost.org/doc/libs/release/libs/context/doc/html/context/architectures.html here].


[endsect]