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<concept id="GUID-25F853BF-BA95-5348-809E-670C04734AB7" xml:lang="en"><title>How

to Choose a Real-time Thread Priority</title><shortdesc>Explains the factors to consider when choosing real time thread

priority.</shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>

<ul>

<li id="GUID-5CF494FB-C569-5B38-AB43-23F489C6F41F"><p> <xref href="GUID-25F853BF-BA95-5348-809E-670C04734AB7.dita#GUID-25F853BF-BA95-5348-809E-670C04734AB7/GUID-80550A0E-CB7D-5988-B07D-4A1542501EF2">Real-time categories</xref> </p> </li>

<li id="GUID-A81CB275-5C5B-5155-821A-ED6E803FC322"><p> <xref href="GUID-25F853BF-BA95-5348-809E-670C04734AB7.dita#GUID-25F853BF-BA95-5348-809E-670C04734AB7/GUID-AEE56C03-9346-54AF-9E80-A00F5856663D">Guidelines for writing real-time code</xref> </p> </li>

</ul>

<section id="GUID-80550A0E-CB7D-5988-B07D-4A1542501EF2"><title>Real-time categories</title> <p>Real-time

tasks divide into two categories: </p> <ul>

<li id="GUID-C773539E-F061-59A0-9777-D41175DF2475"><p> Hard real-time </p> </li>

<li id="GUID-8340689D-F9AB-561F-BAA1-C966EB07F352"><p> Soft real-time </p> </li>

</ul> </section>

<section id="GUID-13F43E08-7E46-4A66-BF36-7516399838DC"><title>Hard real-time</title><p>Hard real-time tasks <i>must</i> complete

within a fixed amount of time otherwise they fail. </p> <p>Audio playback

provides a good example of a hard real-time task. During audio playback the

soundcard driver reads data from an audio buffer to transfer to the soundcard.

The audio application uses a thread to re-fill this buffer before it empties,

and failure to do this before the deadline causes an audible break in the

sound. </p> <p>Telephony applications are another example of hard real-time

tasks. </p> <p>Symbian platform gives no real-time scheduling guarantees to

user processes. Instead, system threads such as those running device drivers

are designed to be efficient and well behaved for their priorities. The following

list gives the estimated deadline ranges other threads are likely to meet

reliably for a given priority on a busy system: </p> <table id="GUID-86D3C2BE-C5B4-5926-91C3-712E37C91968">

<tgroup cols="2"><colspec colname="col0"/><colspec colname="col1"/>

<tbody>

<row>

<entry><p> <b> Deadline range </b>  </p> </entry>

<entry><p> <b> True thread priority value</b>  </p> </entry>

</row>

<row>

<entry><p>&gt; 100ms </p> </entry>

<entry><p>24-26 </p> </entry>

</row>

<row>

<entry><p>&gt; 20ms </p> </entry>

<entry><p>27 </p> </entry>

</row>

<row>

<entry><p>2-20ms </p> </entry>

<entry><p>28-31 </p> </entry>

</row>

</tbody>

</tgroup>

</table> <p>These figures rely on real-time threads completing their work

in less time than the deadline itself, otherwise other threads may fail to

meet their deadlines. These deadline ranges appear conservative because they

are based on what is a achievable on a busy system. Use of lower <i>true</i> priorities

than those in the above table are recommended whenever the implications of

failing to meet a deadline are not very serious and the incidence of failure

is acceptably low in practical testing. </p> </section>

<section id="GUID-AB70E058-6910-4ECB-A84C-1639D56381C9"><title>Soft real-time</title><p>Soft real-time tasks <i>try</i> to

complete within a fixed amount of time. In contrast to hard real-time, exceeding

this time is not considered a failure, although it may affect the user experience

in some way. </p> <p>Soft real-time tasks generally have a quality of service

target expressed as a percentage that is less than 100% (where 100% represents

hard real-time). This value is the percentage of tasks that must complete

within the deadline to satisfy the expected service levels. Usually the programmer

implements a recovery strategy to cope with a thread missing the deadline. </p> <p>Examples

of soft real-time tasks include the responsiveness of the system to user events

and communications protocols. In the case of responsiveness to user events,

an arbitrary deadline is aimed for that is ‘close enough to instantaneous’;

in the case of communication protocols, failure to process data in time causes

it to be retransmitted – possibly at extra cost to the user – but is otherwise

not a catastrophic failure. </p> </section>

<section id="GUID-AEE56C03-9346-54AF-9E80-A00F5856663D"><title>Guidelines

for writing real-time code</title> <p>When developing real-time code follow

these rules: </p> <ul>

<li id="GUID-23007425-8BA8-5AD3-B7CE-7EEB56FDA701"><p>Do the minimum amount

of work required to meet your deadline at the required priority. </p> </li>

<li id="GUID-DDB7AF0E-9245-5511-A6C0-FCA5B6928E9E"><p>For hard real-time tasks,

use the lowest priority you can to achieve your goals within the task deadline. </p> </li>

<li id="GUID-2732E0E6-5CAA-502A-882D-195F497019BF"><p>Avoid using lower priority,

non real-time services, for example file I/O in high priority threads. This

sacrifices the benefit of using a high priority real-time thread. </p> </li>

<li id="GUID-6EC4A4CB-4EBE-5F5B-BC42-C6A926181FAC"><p>For soft real-time tasks

choose a priority above which no significant performance differences are observed

once the required service level is met. </p> </li>

<li id="GUID-329AD22C-052C-5E81-A79B-8747E2D4586E"><p>For typical applications

that respond to user interaction, soft real-time priorities start at values

based on the <xref href="GUID-1054385C-7168-31ED-89D9-4C33E28836A0.dita"><apiname>EPriorityAbsoluteForegroundNormal</apiname></xref> process

priority, but are more often based on the <xref href="GUID-8995D880-69B7-3C31-891B-7A2C57E372FE.dita"><apiname>EPriorityHigh</apiname></xref> process

priority. </p> </li>

<li id="GUID-EC60209A-BFCF-58E2-BD80-8A1DA754CA96"><p>For tasks that will

use all available CPU by the sheer frequency that the thread is scheduled

(for example, using a very high bandwidth communication channel, or achieving

high/maximum frame rates in an action game), lower priorities should be used;

this includes priorities based on the <xref href="GUID-1054385C-7168-31ED-89D9-4C33E28836A0.dita"><apiname>EPriorityAbsoluteForegroundNormal</apiname></xref> process

priority. </p> </li>

<li id="GUID-8C832884-41EF-5F80-8EAA-347A320D9015"><p>Escalating priorities

for soft real-time tasks into the levels normally used for hard real-time

tasks is dangerous. </p> </li>

</ul> </section>

</conbody></concept>