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<concept id="GUID-8EAB97D5-267A-58CC-BC3E-5CB1D72CB7ED" xml:lang="en"><title>Non

pre-emptive servicing</title><shortdesc>This document describes how requests to active objects are serviced

non pre-emptively.</shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>

<p>The UI server frameworks spend most of their time waiting for some event

to occur, and then spend a <i>short</i> time executing a <codeph>RunL()</codeph> function

to service that event. When one event has been serviced, processing returns

to the wait loop in the active scheduler allowing other events to be serviced.</p>

<p>If a second request completes while the <codeph>RunL()</codeph> function

is handling completion of the first request, then another call to <codeph>RunL()</codeph> to

handle completion of the second request cannot be scheduled until the first

invocation of <codeph>RunL()</codeph> is complete.</p>

<p>This means that active object scheduling is non pre-emptive.</p>

<p>Active objects are queued from the active scheduler in order of priority.

The priority is usually determined at the time that the active object is constructed,

although it can be changed later.</p>

<p>If several requests complete while a <codeph>RunL()</codeph> function is

handling the completion of earlier request, the active scheduler takes the

first active object (i.e. the one with the highest priority) with a completed

request and calls its <codeph>RunL()</codeph> function.</p>

<p>As scheduling is non pre-emptive, a high-priority request cannot be serviced

until the previously running request’s <codeph>RunL()</codeph> has completed.</p>

<section id="GUID-55AEDDD4-72FB-4865-90AF-D4F9C5B47582"><title>Recommendations for active object priority</title> <p>Most

active objects should have a priority of <codeph>TPriority::EPriorityStandard</codeph> and

processing should not depend on the order in which active objects are scheduled.</p> <p>However,

any active object involved in user input should have a higher priority over

other active objects to ensure responsiveness and to allow user control over

a device.</p> <p>Long running and background services should have a low priority.</p> </section>

</conbody></concept>