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I was asked the other day what's event loop and how libvirt uses it. Well, I haven't found any good sources on the Internet so I thought of writing the documentation on my own. Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
107 lines
4.4 KiB
XML
107 lines
4.4 KiB
XML
<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
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<html xmlns="http://www.w3.org/1999/xhtml">
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<body>
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<h1>Libvirt's event loop</h1>
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<ul id="toc"></ul>
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<p>
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This page describes the event loop approach used in
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libvirt. Both server and client.
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</p>
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<h2><a name="event_loop">Event driven programming</a></h2>
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<p>Traditionally, a program simply ran once, then terminated.
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This type of program was very common in the early days of
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computing, and lacked any form of user interactivity. This is
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still used frequently, particularly in small one purpose
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programs.</p>
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<p>However, that approach is not suitable for all the types
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of applications. For instance graphical applications spend
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most of their run time waiting for an input from user. Only
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after it happened (in our example a button was clicked, a key
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pressed, etc.) an event is generated to which they respond
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by executing desired function. If generalized, this is how
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many long running programs (daemons) work. Even those who are
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not waiting for direct user input and have no graphical
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interface. Such as Libvirt.</p>
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<img alt="event loop" src="http://libvirt.org/git/?p=libvirt-media.git;a=blob_plain;f=png/event_loop_simple.png;hb=HEAD"/>
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<p>In Libvirt this approach is used in combination with
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<code>poll(2)</code> as all the communication with its
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clients (and domains it manages too) happens through sockets.
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Therefore whenever new client connects, it is given exclusive
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file descriptor which is then watched for incoming events,
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e.g. messages. </p>
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<h2><a name="api">The event loop API</a></h2>
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<p>To work with event loop from our code we have plenty of
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APIs.</p>
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<ul>
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<li><code>virEventAddHandle</code>: Registers a
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callback for monitoring file handle events.</li>
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<li><code>virEventUpdateHandle</code>: Change set of events
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monitored file handle is being watched for.</li>
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<li><code>virEventRemoveHandle</code>: Unregisters
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previously registered file handle so that it is no
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longer monitored for any events.</li>
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<li><code>virEventAddTimeout</code>: Registers a
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callback for timer event.</li>
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<li><code>virEventUpdateTimeout</code>: Changes frequency
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for a timer.</li>
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<li><code>virEventRemoveTimeout</code>: Unregisters
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a timer.</li>
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</ul>
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<p>For more information on these APIs continue reading <a
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href="../html/libvirt-libvirt-event.html">here</a>.</p>
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<h2><a name="worker_pool">Worker pool</a></h2>
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<p>Looking back at the image above we can see one big
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limitation. While processing a message event loop is blocked
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and for an outside observer unresponsive. This is not
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acceptable for Libvirt. Therefore we have came up with the
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following solution.</p>
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<img alt="event loop" src="http://libvirt.org/git/?p=libvirt-media.git;a=blob_plain;f=png/event_loop_worker.png;hb=HEAD"/>
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<p>The event loop does only necessary minimum and hand over
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message processing to another thread. In fact, there can be
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as many processing threads as configured increasing
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processing power.</p>
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<p>To break this high level description into smaller pieces,
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here is what happens when user calls an API:</p>
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<ol>
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<li>User (or management application) calls a Libvirt API.
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Depending on the connection URI, this may or may not
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involve server. Well, for the sake of our
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demonstration we assume the former.</li>
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<li>Remote driver encodes the API among it's arguments
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into an <a href="rpc.html">RPC message</a> and sends
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it to the server.</li>
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<li>Here, server is waiting in <code>poll(2)</code> for
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an event, like incoming message.</li>
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<li>As soon as the first bytes of message are received,
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even loop wakes up and server starts reading the
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whole message.</li>
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<li>Once fully read, the event loop notifies threads
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known as worker threads from which one picks the incoming
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message, decodes and process it.</li>
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<li>As soon as API execution is finished, a reply is sent
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to the client.</li>
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</ol>
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<p>In case that there's no free worker to process an incoming
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message in step 5, message is placed at the end of a message
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queue and is processed in next iteration.</p>
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</body>
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</html>
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