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<h2><a href="https://wiki.asterisk.org/wiki/display/TOP/Thread+pool+and+Work+Queues">Thread pool and Work Queues</a></h2>
<h4>Page <b>added</b> by <a href="https://wiki.asterisk.org/wiki/display/~beagles">Brent Eagles</a>
</h4>
<br/>
<div class="notificationGreySide">
<div class='panelMacro'><table class='warningMacro'><colgroup><col width='24'><col></colgroup><tr><td valign='top'><img src="/wiki/images/icons/emoticons/forbidden.gif" width="16" height="16" align="absmiddle" alt="" border="0"></td><td><b>In progress</b><br /></td></tr></table></div>
<p><font color="white"><br/>
<div class="panel" style="background-color: #7788FF;border-width: 1px;"><div class="panelHeader" style="border-bottom-width: 1px;background-color: #2022FF;"><b>Thread Pools and Work Queues</b></div><div class="panelContent" style="background-color: #7788FF;">
<ul>
<li>Rationale:
<ul>
<li>Reduce complexity of creating robust multithreaded applications.</li>
<li>XXX more here!!!!</li>
</ul>
</li>
<li>How:
<ul>
<li>Serialize access to shared data, by serializing the operations themselves. That is, don't share the data.</li>
</ul>
</li>
<li>Wha?
<ul>
<li>Okay.. it breaks down something like this...</li>
</ul>
</li>
</ul>
</div></div></font></p>
<p><font color="white"><br/>
<div class="panel" style="background-color: #7788FF;border-width: 1px;"><div class="panelHeader" style="border-bottom-width: 1px;background-color: #2022FF;"><b>Thread Pools</b></div><div class="panelContent" style="background-color: #7788FF;">
<ul>
<li>Familiar concept. A managed collection of threads for performing work.</li>
<li>Asterisk SCF thread pool details.
<ul>
<li>Resizable, with an API for setting objects that implement custom sizing policies.</li>
<li>Default implementation cleans up idle threads to recover resources after peaks.</li>
<li>Each pools owns a work queue.</li>
</ul>
</li>
<li>What is a work queue?</li>
</ul>
</div></div></font></p>
<p><font color="white"><br/>
<div class="panel" style="background-color: #7788FF;border-width: 1px;"><div class="panelHeader" style="border-bottom-width: 1px;background-color: #2022FF;"><b>Work Queues</b></div><div class="panelContent" style="background-color: #7788FF;">
<ul>
<li>A work queue is a collection of "Work", which is basically an object that implements the Work interface.</li>
<li>Here is the kicker. A Work object may itself be a collection of Work objects.</li>
<li>Voila, we have a mechanism for queuing work for a set of data in a way that it will only ever be acted on by a single thread at a time.</li>
<li>Implications:
<ul>
<li>All Ice method and methods that respond to potentially concurrent events must be implemented in terms of Work object instances.</li>
<li>For Ice, AMI and AMD tends to get used a lot.</li>
<li>Code is organized differently. A little awkward at first but quickly feels natural.</li>
</ul>
</li>
</ul>
</div></div></font></p>
<p><font color="white"><br/>
<div class="panel" style="background-color: #7788FF;border-width: 1px;"><div class="panelHeader" style="border-bottom-width: 1px;background-color: #2022FF;"><b>How Does it Work?</b></div><div class="panelContent" style="background-color: #7788FF;">
<ul>
<li>Basically for any object that has methods that may be called by multiple concurrent threads of execution, organize it's associated state and implement the implementation details in terms of a "work" object and put it on a queue dedicated to that object.</li>
<li>A thread pool worker thread obtains a work item from its queue, and this work item is actually a queue of items belonging to your object.</li>
<li>The worker thread "executes" the work item, ultimately executing your queued work item.</li>
<li>As the work items for a single object are in a single work queue item and that work queue item may only be processed by a single thread, you have serialized access without mutexes and without concern of race conditions and deadlock.</li>
<li>Locks or no locks, it is a great way to avoid race conditions by organizing data access.</li>
</ul>
</div></div></font></p>
<p><font color="white"><br/>
<div class="panel" style="background-color: #7788FF;border-width: 1px;"><div class="panelHeader" style="border-bottom-width: 1px;background-color: #2022FF;"><b>Anatomy of a Work Item</b></div><div class="panelContent" style="background-color: #7788FF;">
<div class="code panel" style="border-width: 1px;"><div class="codeContent panelContent">
<pre class="theme: Confluence; brush: java; gutter: false">class MyWorkItem : public AsteriskSCF::System::WorkQueue::V1::Work
{
public:
MyWorkItem(const StateBeingWorkedOnPtr& state) :
mState(state)
{ }
void execute()
{ /* Do some stuff to state, possibly enqueuing another work item */ }
};</pre>
</div></div>
<p><em>a graphic would be handy</em></p>
</div></div></font></p>
<p><font color="white"><br/>
<div class="panel" style="background-color: #7788FF;border-width: 1px;"><div class="panelHeader" style="border-bottom-width: 1px;background-color: #2022FF;"><b>In the Wild</b></div><div class="panelContent" style="background-color: #7788FF;">
<ul>
<li>SIP Session Gateway.</li>
</ul>
</div></div></font></p>
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