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de4b03cb21
Add a link to the section "No Spurious Wakeups" in the condition_variable section. Make a pass over "migrating fibers between threads," adding a link to the whole new section on that same topic.
98 lines
3.8 KiB
Plaintext
98 lines
3.8 KiB
Plaintext
[/
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Copyright Oliver Kowalke 2013.
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Distributed under the Boost Software License, Version 1.0.
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(See accompanying file LICENSE_1_0.txt or copy at
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http://www.boost.org/LICENSE_1_0.txt
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]
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[section:rationale Rationale]
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[heading distinction between coroutines and fibers]
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The fiber library extends the coroutine library by adding a scheduler and
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synchronization mechanisms.
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* a coroutine yields
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* a fiber blocks
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When a coroutine yields, it passes control directly to its caller (or, in the
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case of symmetric coroutines, a designated other coroutine). When a fiber
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blocks, it implicitly passes control to the fiber scheduler. Coroutines have
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no scheduler because they need no scheduler.[footnote
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[@http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n4024.pdf 'N4024:
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Distinguishing coroutines and fibers']].
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[heading what about transactional memory]
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GCC supports transactional memory since version 4.7. Unfortunately tests show
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that transactional memory is slower (ca. 4x) than spinlocks using atomics.
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Once transactional memory is improved (supporting hybrid tm), spinlocks will
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be replaced by __transaction_atomic{} statements surrounding the critical
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sections.
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[heading synchronization between fibers running in different threads]
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Synchronization classes from __boost_thread__ block the entire thread. In
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contrast, the synchronization classes from __boost_fiber__ block only specific
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fibers, so that the scheduler can still keep the thread busy running other
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fibers in the meantime.
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The synchronization classes from __boost_fiber__ are designed to be
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thread-safe, i.e. it is possible to synchronize fibers running in different
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threads as well as fibers running in the same thread. (However, there is a
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build option to disable cross-thread fiber synchronization support; see [link
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cross_thread_sync this description].)
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[#spurious_wakeup]
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[heading spurious wakeup]
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Spurious wakeup can happen when using
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[@http://en.cppreference.com/w/cpp/thread/condition_variable
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`std::condition_variable`]: the condition variable appears to be have been
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signaled while the awaited condition may still be false. Spurious wakeup can
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happen repeatedly and is caused on some multiprocessor systems where making
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`std::condition_variable` wakeup completely predictable would slow down all
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`std::condition_variable` operations.[footnote David R. Butenhof ["Programming
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with POSIX Threads]]
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__condition__ is not subject to spurious wakeup. Nonetheless it is
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prudent to test the business-logic condition in a `wait()` loop [mdash] or,
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equivalently, use one of the `wait( lock, predicate )` overloads.
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See also [link condition_variable_spurious_wakeups No Spurious Wakeups].
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[heading migrating fibers between threads]
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Support for migrating fibers between threads has been integrated. The
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user-defined scheduler must call __migrate__ on a fiber-context on the
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destination thread, passing `migrate()` the fiber-context to migrate. (For
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more information about custom schedulers, see [link custom Customization].)
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Examples `work_sharing` and `work_stealing` in directory `examples` might be
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used as a blueprint.
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See also [link migration Migrating fibers between threads].
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[heading support for Boost.Asio]
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Support for __boost_asio__'s __async_result__ is not part of the official API.
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However, to integrate with a `boost::asio::io_service`, see [link integration
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Sharing a Thread with Another Main Loop]. To interface smoothly with an
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arbitrary Asio async I/O operation, see [link callbacks_asio Then There's
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__boost_asio__].
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[heading tested compilers]
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The library was tested with GCC-5.1.1, Clang-3.6.0 and MSVC-14.0 in c++11-mode.
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[heading supported architectures]
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__boost_fiber__ depends on __boost_context__ - the list of supported architectures
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can be found [@http://www.boost.org/doc/libs/release/libs/context/doc/html/context/architectures.html here].
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[endsect]
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