Cory McWilliams
f421606e21
git-svn-id: https://www.unprompted.com/svn/projects/tildefriends/trunk@4308 ed5197a5-7fde-0310-b194-c3ffbd925b24
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6.3 KiB
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142 lines
6.3 KiB
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.. _design:
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Design overview
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===============
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libuv is cross-platform support library which was originally written for `Node.js`_. It's designed
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around the event-driven asynchronous I/O model.
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.. _Node.js: https://nodejs.org
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The library provides much more than a simple abstraction over different I/O polling mechanisms:
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'handles' and 'streams' provide a high level abstraction for sockets and other entities;
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cross-platform file I/O and threading functionality is also provided, amongst other things.
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Here is a diagram illustrating the different parts that compose libuv and what subsystem they
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relate to:
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.. image:: static/architecture.png
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:scale: 75%
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:align: center
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Handles and requests
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^^^^^^^^^^^^^^^^^^^^
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libuv provides users with 2 abstractions to work with, in combination with the event loop:
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handles and requests.
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Handles represent long-lived objects capable of performing certain operations while active. Some examples:
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- A prepare handle gets its callback called once every loop iteration when active.
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- A TCP server handle that gets its connection callback called every time there is a new connection.
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Requests represent (typically) short-lived operations. These operations can be performed over a
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handle: write requests are used to write data on a handle; or standalone: getaddrinfo requests
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don't need a handle they run directly on the loop.
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The I/O loop
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^^^^^^^^^^^^
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The I/O (or event) loop is the central part of libuv. It establishes the content for all I/O
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operations, and it's meant to be tied to a single thread. One can run multiple event loops
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as long as each runs in a different thread. The libuv event loop (or any other API involving
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the loop or handles, for that matter) **is not thread-safe** except where stated otherwise.
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The event loop follows the rather usual single threaded asynchronous I/O approach: all (network)
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I/O is performed on non-blocking sockets which are polled using the best mechanism available
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on the given platform: epoll on Linux, kqueue on OSX and other BSDs, event ports on SunOS and IOCP
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on Windows. As part of a loop iteration the loop will block waiting for I/O activity on sockets
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which have been added to the poller and callbacks will be fired indicating socket conditions
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(readable, writable hangup) so handles can read, write or perform the desired I/O operation.
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In order to better understand how the event loop operates, the following diagram illustrates all
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stages of a loop iteration:
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.. image:: static/loop_iteration.png
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:scale: 75%
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:align: center
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#. The loop concept of 'now' is initially set.
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#. Due timers are run if the loop was run with ``UV_RUN_DEFAULT``. All active timers scheduled
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for a time before the loop's concept of *now* get their callbacks called.
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#. If the loop is *alive* an iteration is started, otherwise the loop will exit immediately. So,
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when is a loop considered to be *alive*? If a loop has active and ref'd handles, active
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requests or closing handles it's considered to be *alive*.
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#. Pending callbacks are called. All I/O callbacks are called right after polling for I/O, for the
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most part. There are cases, however, in which calling such a callback is deferred for the next
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loop iteration. If the previous iteration deferred any I/O callback it will be run at this point.
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#. Idle handle callbacks are called. Despite the unfortunate name, idle handles are run on every
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loop iteration, if they are active.
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#. Prepare handle callbacks are called. Prepare handles get their callbacks called right before
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the loop will block for I/O.
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#. Poll timeout is calculated. Before blocking for I/O the loop calculates for how long it should
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block. These are the rules when calculating the timeout:
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* If the loop was run with the ``UV_RUN_NOWAIT`` flag, the timeout is 0.
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* If the loop is going to be stopped (:c:func:`uv_stop` was called), the timeout is 0.
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* If there are no active handles or requests, the timeout is 0.
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* If there are any idle handles active, the timeout is 0.
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* If there are any handles pending to be closed, the timeout is 0.
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* If none of the above cases matches, the timeout of the closest timer is taken, or
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if there are no active timers, infinity.
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#. The loop blocks for I/O. At this point the loop will block for I/O for the duration calculated
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in the previous step. All I/O related handles that were monitoring a given file descriptor
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for a read or write operation get their callbacks called at this point.
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#. Check handle callbacks are called. Check handles get their callbacks called right after the
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loop has blocked for I/O. Check handles are essentially the counterpart of prepare handles.
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#. Close callbacks are called. If a handle was closed by calling :c:func:`uv_close` it will
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get the close callback called.
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#. The loop concept of 'now' is updated.
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#. Due timers are run. Note that 'now' is not updated again until the next loop iteration.
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So if a timer became due while other timers were being processed, it won't be run until
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the following event loop iteration.
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#. Iteration ends. If the loop was run with ``UV_RUN_NOWAIT`` or ``UV_RUN_ONCE`` modes the
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iteration ends and :c:func:`uv_run` will return. If the loop was run with ``UV_RUN_DEFAULT``
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it will continue from the start if it's still *alive*, otherwise it will also end.
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.. important::
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libuv uses a thread pool to make asynchronous file I/O operations possible, but
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network I/O is **always** performed in a single thread, each loop's thread.
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.. note::
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While the polling mechanism is different, libuv makes the execution model consistent
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across Unix systems and Windows.
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File I/O
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^^^^^^^^
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Unlike network I/O, there are no platform-specific file I/O primitives libuv could rely on,
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so the current approach is to run blocking file I/O operations in a thread pool.
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For a thorough explanation of the cross-platform file I/O landscape, check out
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`this post <https://blog.libtorrent.org/2012/10/asynchronous-disk-io/>`_.
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libuv currently uses a global thread pool on which all loops can queue work. 3 types of
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operations are currently run on this pool:
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* File system operations
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* DNS functions (getaddrinfo and getnameinfo)
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* User specified code via :c:func:`uv_queue_work`
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.. warning::
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See the :c:ref:`threadpool` section for more details, but keep in mind the thread pool size
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is quite limited.
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