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    10 <!DOCTYPE concept

    11   PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">

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

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

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

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

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

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

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

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

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

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

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

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

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

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

    26 although it can be changed later.</p>

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

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

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

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

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

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

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

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

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

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

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

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

    39 </conbody></concept>