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[ceph.git] / ceph / src / boost / libs / fiber / doc / overview.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:overview Overview]

__boost_fiber__  provides a framework for micro-/userland-threads (fibers)
scheduled cooperatively.
The API contains classes and functions to manage and synchronize fibers similiarly to
__std_thread__.

Each fiber has its own stack.

A fiber can save the current execution state, including all registers
and CPU flags, the instruction pointer, and the stack pointer and later restore
this state.
The idea is to have multiple execution paths running on a single thread using
cooperative scheduling (versus threads, which are preemptively scheduled). The
running fiber decides explicitly when it should yield to allow another fiber to
run (context switching).
__boost_fiber__ internally uses __econtext__ from __boost_context__; the classes in
this library manage, schedule and, when needed, synchronize those execution contexts.
A context switch between threads usually costs thousands of CPU cycles on x86,
compared to a fiber switch with less than a hundred cycles.
A fiber runs on a single thread at any point in time.

In order to use the classes and functions described here, you can either include
the specific headers specified by the descriptions of each class or function, or
include the master library header:

    #include <boost/fiber/all.hpp>

which includes all the other headers in turn.

The namespaces used are:

    namespace boost::fibers
    namespace boost::this_fiber

[heading Fibers and Threads]

Control is cooperatively passed between fibers launched on a given thread. At
a given moment, on a given thread, at most one fiber is running.

Spawning additional fibers on a given thread does not distribute your program
across more hardware cores, though it can make more effective use of the core
on which it's running.

On the other hand, a fiber may safely access any resource exclusively owned by
its parent thread without explicitly needing to defend that resource against
concurrent access by other fibers on the same thread. You are already
guaranteed that no other fiber on that thread is concurrently touching that
resource. This can be particularly important when introducing concurrency in
legacy code. You can safely spawn fibers running old code, using asynchronous
I/O to interleave execution.

In effect, fibers provide a natural way to organize concurrent code based on
asynchronous I/O. Instead of chaining together completion handlers, code
running on a fiber can make what looks like a normal blocking function call.
That call can cheaply suspend the calling fiber, allowing other fibers on the
same thread to run. When the operation has completed, the suspended fiber
resumes, without having to explicitly save or restore its state. Its local
stack variables persist across the call.

A fiber can be migrated from one thread to another, though the library does
not do this by default. It is possible for you to supply a custom scheduler
that migrates fibers between threads. You may specify custom fiber properties
to help your scheduler decide which fibers are permitted to migrate. Please
see [link migration Migrating fibers between threads] and [link custom
Customization] for more details.

A fiber launched on a particular thread continues running on that thread
unless migrated. It might be unblocked (see [link blocking Blocking] below) by
some other thread, but that only transitions the fiber from ["blocked] to
["ready] on its current thread [mdash] it does not cause the fiber to
resume on the thread that unblocked it.

[#thread_local_storage]
[heading thread-local storage]
Unless migrated, a fiber may access thread-local storage; however that storage
will be shared among all fibers running on the same thread. For fiber-local
storage, please see __fsp__.

[#cross_thread_sync]
[heading BOOST_FIBERS_NO_ATOMICS]
The fiber synchronization objects provided by this library will, by default,
safely synchronize fibers running on different threads. However, this level of
synchronization can be removed (for performance) by building the library with
[*`BOOST_FIBERS_NO_ATOMICS`] defined. When the library is built with that macro,
you must ensure that all the fibers referencing a particular synchronization
object are running in the same thread. Please see [link synchronization
Synchronization].

[#blocking]
[heading Blocking]

Normally, when this documentation states that a particular fiber ['blocks] (or
equivalently, ['suspends),] it means that it yields control, allowing other
fibers on the same thread to run. The synchronization mechanisms provided by
__boost_fiber__ have this behavior.

A fiber may, of course, use normal thread synchronization mechanisms; however
a fiber that invokes any of these mechanisms will block its entire thread,
preventing any other fiber from running on that thread in the meantime. For
instance, when a fiber wants to wait for a value from another fiber in the
same thread, using `std::future` would be unfortunate: `std::future::get()` would
block the whole thread, preventing the other fiber from delivering its value.
Use __future__ instead.

Similarly, a fiber that invokes a normal blocking I/O operation will block its
entire thread. Fiber authors are encouraged to consistently use asynchronous
I/O. __boost_asio__ and other asynchronous I/O operations can
straightforwardly be adapted for __boost_fiber__: see [link callbacks
Integrating Fibers with Asynchronous Callbacks].

__boost_fiber__ depends upon __boost_context__.
Boost version 1.61.0 or greater is required.

[note This library requires C++11!]


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