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diff --git a/3rdparty/libuv/docs/src/design.rst b/3rdparty/libuv/docs/src/design.rst new file mode 100644 index 00000000000..34c3cff68e5 --- /dev/null +++ b/3rdparty/libuv/docs/src/design.rst @@ -0,0 +1,137 @@ + +.. _design: + +Design overview +=============== + +libuv is cross-platform support library which was originally written for NodeJS. It's designed +around the event-driven asynchronous I/O model. + +The library provides much more than simply abstraction over different I/O polling mechanisms: +'handles' and 'streams' provide a high level abstraction for sockets and other entities; +cross-platform file I/O and threading functionality is also provided, amongst other things. + +Here is a diagram illustrating the different parts that compose libuv and what subsystem they +relate to: + +.. image:: static/architecture.png + :scale: 75% + :align: center + + +Handles and requests +^^^^^^^^^^^^^^^^^^^^ + +libuv provides users with 2 abstractions to work with, in combination with the event loop: +handles and requests. + +Handles represent long-lived objects capable of performing certain operations while active. Some +examples: a prepare handle gets its callback called once every loop iteration when active, and +a TCP server handle get its connection callback called every time there is a new connection. + +Requests represent (typically) short-lived operations. These operations can be performed over a +handle: write requests are used to write data on a handle; or standalone: getaddrinfo requests +don't need a handle they run directly on the loop. + + +The I/O loop +^^^^^^^^^^^^ + +The I/O (or event) loop is the central part of libuv. It establishes the content for all I/O +operations, and it's meant to be tied to a single thread. One can run multiple event loops +as long as each runs in a different thread. The libuv event loop (or any other API involving +the loop or handles, for that matter) **is not thread-safe** except where stated otherwise. + +The event loop follows the rather usual single threaded asynchronous I/O approach: all (network) +I/O is performed on non-blocking sockets which are polled using the best mechanism available +on the given platform: epoll on Linux, kqueue on OSX and other BSDs, event ports on SunOS and IOCP +on Windows. As part of a loop iteration the loop will block waiting for I/O activity on sockets +which have been added to the poller and callbacks will be fired indicating socket conditions +(readable, writable hangup) so handles can read, write or perform the desired I/O operation. + +In order to better understand how the event loop operates, the following diagram illustrates all +stages of a loop iteration: + +.. image:: static/loop_iteration.png + :scale: 75% + :align: center + + +#. The loop concept of 'now' is updated. The event loop caches the current time at the start of + the event loop tick in order to reduce the number of time-related system calls. + +#. If the loop is *alive* an iteration is started, otherwise the loop will exit immediately. So, + when is a loop considered to be *alive*? If a loop has active and ref'd handles, active + requests or closing handles it's considered to be *alive*. + +#. Due timers are run. All active timers scheduled for a time before the loop's concept of *now* + get their callbacks called. + +#. Pending callbacks are called. All I/O callbacks are called right after polling for I/O, for the + most part. There are cases, however, in which calling such a callback is deferred for the next + loop iteration. If the previous iteration deferred any I/O callback it will be run at this point. + +#. Idle handle callbacks are called. Despite the unfortunate name, idle handles are run on every + loop iteration, if they are active. + +#. Prepare handle callbacks are called. Prepare handles get their callbacks called right before + the loop will block for I/O. + +#. Poll timeout is calculated. Before blocking for I/O the loop calculates for how long it should + block. These are the rules when calculating the timeout: + + * If the loop was run with the ``UV_RUN_NOWAIT`` flag, the timeout is 0. + * If the loop is going to be stopped (:c:func:`uv_stop` was called), the timeout is 0. + * If there are no active handles or requests, the timeout is 0. + * If there are any idle handles active, the timeout is 0. + * If there are any handles pending to be closed, the timeout is 0. + * If none of the above cases was matched, the timeout of the closest timer is taken, or + if there are no active timers, infinity. + +#. The loop blocks for I/O. At this point the loop will block for I/O for the timeout calculated + on the previous step. All I/O related handles that were monitoring a given file descriptor + for a read or write operation get their callbacks called at this point. + +#. Check handle callbacks are called. Check handles get their callbacks called right after the + loop has blocked for I/O. Check handles are essentially the counterpart of prepare handles. + +#. Close callbacks are called. If a handle was closed by calling :c:func:`uv_close` it will + get the close callback called. + +#. Special case in case the loop was run with ``UV_RUN_ONCE``, as it implies forward progress. + It's possible that no I/O callbacks were fired after blocking for I/O, but some time has passed + so there might be timers which are due, those timers get their callbacks called. + +#. Iteration ends. If the loop was run with ``UV_RUN_NOWAIT`` or ``UV_RUN_ONCE`` modes the + iteration is ended and :c:func:`uv_run` will return. If the loop was run with ``UV_RUN_DEFAULT`` + it will continue from the start if it's still *alive*, otherwise it will also end. + + +.. important:: + libuv uses a thread pool to make asynchronous file I/O operations possible, but + network I/O is **always** performed in a single thread, each loop's thread. + +.. note:: + While the polling mechanism is different, libuv makes the execution model consistent + across Unix systems and Windows. + + +File I/O +^^^^^^^^ + +Unlike network I/O, there are no platform-specific file I/O primitives libuv could rely on, +so the current approach is to run blocking file I/O operations in a thread pool. + +For a thorough explanation of the cross-platform file I/O landscape, checkout +`this post <http://blog.libtorrent.org/2012/10/asynchronous-disk-io/>`_. + +libuv currently uses a global thread pool on which all loops can queue work on. 3 types of +operations are currently run on this pool: + + * Filesystem operations + * DNS functions (getaddrinfo and getnameinfo) + * User specified code via :c:func:`uv_queue_work` + +.. warning:: + See the :c:ref:`threadpool` section for more details, but keep in mind the thread pool size + is quite limited. |