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<concept id="GUID-A3EE649B-FF95-539F-8037-54706E4702D5" xml:lang="en"><title>How

to Choose Thread Priority</title><shortdesc>Explains the factors to consider when choosing a thread priority.</shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>

<p>The two main considerations when choosing the priority of a thread are:

the timing requirements of the task that the thread must accomplish, and the

other tasks that are likely to be running at the same time. </p>

<ul>

<li id="GUID-53666CA1-A159-595C-A077-418C161670CB"><p> <xref href="GUID-A3EE649B-FF95-539F-8037-54706E4702D5.dita#GUID-A3EE649B-FF95-539F-8037-54706E4702D5/GUID-1E013283-E81C-5144-8055-5667215A3D00">Choosing a process priority</xref> </p> </li>

<li id="GUID-D592C147-7A98-5722-B345-7DECC98F0B94"><p> <xref href="GUID-A3EE649B-FF95-539F-8037-54706E4702D5.dita#GUID-A3EE649B-FF95-539F-8037-54706E4702D5/GUID-4248FC3D-8901-5B16-9ACB-9F96830875A6">Choosing the priority of a thread relative to a process</xref> </p> </li>

<li id="GUID-B11798AA-E313-5E1F-94A9-F15729744DFC"><p> <xref href="GUID-A3EE649B-FF95-539F-8037-54706E4702D5.dita#GUID-A3EE649B-FF95-539F-8037-54706E4702D5/GUID-255A62AC-AE94-5296-9B23-87414F68BC0D">Choosing a process-independent thread priority</xref> </p> </li>

<li id="GUID-133625B3-1DBB-5937-908B-1FEDDD99C79A"><p> <xref href="GUID-A3EE649B-FF95-539F-8037-54706E4702D5.dita#GUID-A3EE649B-FF95-539F-8037-54706E4702D5/GUID-D11BCB87-CC0F-50EF-AB81-F3E46C6C4081">Guideline summary</xref> </p> </li>

<li id="GUID-389CD1F8-D39D-599E-9582-AA052DA99AC3"><p> <xref href="GUID-A3EE649B-FF95-539F-8037-54706E4702D5.dita#GUID-A3EE649B-FF95-539F-8037-54706E4702D5/GUID-357FC034-EEB4-5D9F-B51A-E6A1400247AD">Long running background tasks</xref> </p> </li>

<li id="GUID-21950134-8365-576C-AC8F-FD1BE93EB8DF"><p> <xref href="GUID-A3EE649B-FF95-539F-8037-54706E4702D5.dita#GUID-A3EE649B-FF95-539F-8037-54706E4702D5/GUID-86CD6AE8-33E2-549D-8B51-DE705CE017E7">Special timing requirements</xref> </p> </li>

</ul>

<section id="GUID-1E013283-E81C-5144-8055-5667215A3D00"><title>Choosing a

process priority</title> <p>Choose one of the four standard process priorities

listed below as the <i>basis</i> for a <i>range of relative priorities</i>: </p> <ol id="GUID-D1020288-02AD-5BDD-A3EA-F7716EB7A941">

<li id="GUID-D87CB980-CBAC-5302-AB28-92ACBED89A8C"><p> <codeph>EPriorityForeground</codeph>  </p> <p>This

is the default process priority. Use this priority for: </p> <ul>

<li id="GUID-4354C28F-9929-5CEA-B515-E235CD90C961"><p>The currently selected

user interface application which appears in the foreground on the device screen.

This application is said to have the <i>focus</i>. </p> </li>

<li id="GUID-8EE56372-79AB-5841-A2FC-B87282F61CD3"><p>Servers that do not

have specific timing or response requirements. </p> </li>

</ul> <p>The UIKON framework calls <xref href="GUID-C197C9A7-EA05-3F24-9854-542E984C612D.dita#GUID-C197C9A7-EA05-3F24-9854-542E984C612D/GUID-B5FBAB15-4946-3F3F-B03B-C66D6DF573CE"><apiname>User::SetPriorityControl</apiname></xref> <codeph>(ETrue)</codeph> so

that the window server can switch applications to background when not in focus.

This is known as <i>priority control</i>. The application can further specify

how the window server makes the priority change using <xref href="GUID-643DDA78-C7A7-386D-AB3F-8710141DDDA9.dita#GUID-643DDA78-C7A7-386D-AB3F-8710141DDDA9/GUID-E940B930-A882-3624-B8FF-D11626B6EA95"><apiname>RWsSession::ComputeMode()</apiname></xref>.

However, applications should not disable <i>priority control</i> to run permanently

at <codeph>EPriorityForeground</codeph> process priority without strong justification. <i>Priority

control</i> is irrelevant to processes without a user interface, for which

it should remain disabled. </p> </li>

<li id="GUID-CF0DAC60-5149-56BE-A5DF-F9A86C5037B8"><p> <codeph>EPriorityBackground</codeph>  </p> <p>Use

this process priority for: </p> <ul>

<li id="GUID-8B43DFDD-9B1A-52A3-A2B0-C4D4C3067D46"><p>all running user interface

applications except the currently selected application. These applications

appear in the background on the device screen. </p> </li>

<li id="GUID-AA41948D-6929-5C67-A501-3612AF76F750"><p>applications which do

not need to respond rapidly to user actions. For example applications that

do not have a graphical user interface. </p> </li>

</ul> </li>

<li id="GUID-6AF283AE-E3B9-558A-A526-4CFCA79E77C7"><p> <codeph>EPriorityHigh</codeph>  </p> <p>Use

this process priority for system servers which should respond quickly to client

requests. These servers must do minimal work at this priority level before

quitting to allow other threads to run. </p> </li>

<li id="GUID-5C69DA26-9106-50A7-B4C4-55940BFB071B"><p> <codeph>EPriorityLow</codeph>  </p> <p>Use

this priority for processes which can run in the background. </p> </li>

<li id="GUID-4B16E703-2EA0-526D-89A1-D0F59C06BAC1"><p>The remaining priority

values:<codeph>EPriorityWindowServer</codeph>,<codeph>EPriorityFileServer</codeph>, <codeph>EPrioritySupervisor</codeph> and <codeph>EPriorityRealTimeServer</codeph> can only be used by processes with <codeph>ProtServ</codeph> (Protected

Server) Security Capability. </p> </li>

</ol> <p>See <xref href="GUID-7B173E02-9697-51F4-A7E8-795AEF547109.dita#GUID-7B173E02-9697-51F4-A7E8-795AEF547109/GUID-B9022468-F4E7-536B-B1D4-58DAA9EA5914">Process-relative

scheme</xref> in <xref href="GUID-7B173E02-9697-51F4-A7E8-795AEF547109.dita#GUID-7B173E02-9697-51F4-A7E8-795AEF547109/GUID-0194B574-806B-5BA3-B65C-6DC5A80CAB86">Priority

scheme for general user-side threads</xref> in <xref href="GUID-7B173E02-9697-51F4-A7E8-795AEF547109.dita">Thread

priority scheme</xref>. </p> </section>

<section id="GUID-4248FC3D-8901-5B16-9ACB-9F96830875A6"><title>Choosing the

priority of a thread relative to a process</title> <p> <codeph>EPriorityNormal</codeph> is

the default <i>relative</i> thread priority. Choose <codeph>EPriorityLess</codeph> and <codeph>EPriorityMuchLess</codeph> to

set a lower priority than the owning process. Choose <codeph>EPriorityMore</codeph> and <codeph>EPriorityMuchMore</codeph> to

set a higher priority than the owning process. </p> <p>Application programs

and system servers with no <xref href="GUID-A3EE649B-FF95-539F-8037-54706E4702D5.dita#GUID-A3EE649B-FF95-539F-8037-54706E4702D5/GUID-86CD6AE8-33E2-549D-8B51-DE705CE017E7">special

timing requirements</xref> running with a <i>process</i> priority of <codeph>EPriorityForeground</codeph> or <codeph>EPriorityBackground</codeph> should

use the default <i>relative</i> priority <codeph>EPriorityNormal</codeph> for

their main thread. </p> </section>

<section id="GUID-255A62AC-AE94-5296-9B23-87414F68BC0D"><title>Choosing a

process-independent thread priority</title> <p>Use process-independent thread

priorities only when <i>strictly</i> required. Typically, this is when the

thread's behaviour means that its priority must lie outside the range of relative

priorities available with a chosen process priority. Such threads are used

for tasks with <xref href="GUID-A3EE649B-FF95-539F-8037-54706E4702D5.dita#GUID-A3EE649B-FF95-539F-8037-54706E4702D5/GUID-86CD6AE8-33E2-549D-8B51-DE705CE017E7">special

timing requirements</xref>, such as long running background tasks or time

critical tasks. </p> <p>Avoid "absolute" priorities in the middle of the range

of relative priorities available with the chosen process priority. For example,

consider what happens if you use <xref href="GUID-1054385C-7168-31ED-89D9-4C33E28836A0.dita"><apiname>EPriorityAbsoluteForegroundNormal</apiname></xref>.

This is in the middle of the range of priorities relative to the <codeph>EPriorityForeground</codeph> process

priority. If the process priority is changed to <codeph>EPriorityBackground</codeph>,

then the thread will now pre-empt all threads that have been assigned process-relative

priorities. </p> <p>Time critical tasks can be divided into those that must

complete as quickly as possible and those that need to complete within a fixed

time interval. The former are simply high priority tasks, the latter are known

as real-time tasks. See <xref href="GUID-25F853BF-BA95-5348-809E-670C04734AB7.dita">How

to choose a real-time thread priority</xref>. </p> </section>

<section id="GUID-D11BCB87-CC0F-50EF-AB81-F3E46C6C4081"><title>Guideline summary</title> <ul>

<li id="GUID-988EA6D9-3534-565B-80CB-C63B7F808E88"><p>Use process-relative

thread priorities instead of process-independent thread priorities whenever

possible. </p> </li>

<li id="GUID-F5CCDDE0-BB38-5043-87FC-ED7D41826E11"><p>Only use process-independent

thread priorities for threads that need to run at particularly high or low

priorities compared to other threads running on the system. Examples of these

two extremes are short term, time critical tasks or <xref href="GUID-A3EE649B-FF95-539F-8037-54706E4702D5.dita#GUID-A3EE649B-FF95-539F-8037-54706E4702D5/GUID-357FC034-EEB4-5D9F-B51A-E6A1400247AD">long running background tasks</xref>. </p> </li>

<li id="GUID-BC583446-B98E-5D47-910C-5D4E0037D39B"><p>Ensure that threads

with priorities above <xref href="GUID-1054385C-7168-31ED-89D9-4C33E28836A0.dita"><apiname>EPriorityAbsoluteForegroundNormal</apiname></xref> such

as the GUI foreground application and system servers do not perform long-running

tasks. </p> </li>

<li id="GUID-8166180C-05CA-5514-B6D1-7D21DF9B9C57"><p>Ensure that threads

with priorities in the range above <xref href="GUID-CD20880F-21EB-389C-A0A4-8DFFE714B102.dita"><apiname>EPriorityAbsoluteBackgroundNormal</apiname></xref>,

and below or equal to <xref href="GUID-1054385C-7168-31ED-89D9-4C33E28836A0.dita"><apiname>EPriorityAbsoluteForegroundNormal</apiname></xref> do

not perform long-running tasks. </p> </li>

<li id="GUID-697D648E-FE24-5F18-8622-1BAEEDEBB9F9"><p>Threads can have equal

priorities. Higher or lower priorities are only needed if one thread must

pre-empt another. </p> </li>

<li id="GUID-D4753029-3139-59B6-8C29-BFF9FEADB003"><p>GUI applications must

not disable <i>priority control</i> to run permanently at <codeph>EPriorityForeground</codeph> process

priority without very good reason. See also <xref href="GUID-A3EE649B-FF95-539F-8037-54706E4702D5.dita#GUID-A3EE649B-FF95-539F-8037-54706E4702D5/GUID-1E013283-E81C-5144-8055-5667215A3D00">choosing

a process priority</xref> for a description of <i>priority control</i>. </p> </li>

<li id="GUID-56084ABD-AF39-5EB9-8AFE-145CCD2EF55B"><p>Unless there is a specific

requirement, disable priority control for processes without a user interface. </p> </li>

<li id="GUID-4B743232-CFD3-5CEC-9B9D-FDC99CB194A0"><p>Avoid adding unnecessary

extra functionality to high priority threads. Instead, create a new lower

priority thread for this code. Always perform the minimum work at a given

priority to meet a task deadline. </p> </li>

<li id="GUID-3C3C4B91-28DC-5083-B80C-77F54C06B7E3"><p>Never assume a higher

priority thread will run before a lower priority thread. The scheduling algorithm

and the <xref href="GUID-7B173E02-9697-51F4-A7E8-795AEF547109.dita#GUID-7B173E02-9697-51F4-A7E8-795AEF547109/GUID-F5B98A28-EEDA-58BD-A6CC-62C1621D44DF">thread

priority value mapping table</xref> might change. Use a synchronization mechanism

such as a semaphore or mutex to guarantee that a thread is not interrupted. </p> </li>

<li id="GUID-5FFBE78D-511C-5DAF-A410-BDCC4F8AC57B"><p>Always avoid unnecessary <xref href="GUID-A3EE649B-FF95-539F-8037-54706E4702D5.dita#GUID-A3EE649B-FF95-539F-8037-54706E4702D5/GUID-357FC034-EEB4-5D9F-B51A-E6A1400247AD">long

running background tasks</xref> as this reduces battery performance. </p> </li>

<li id="GUID-60AA26FC-A170-50B1-8F7E-27FC4718220A"><p>Devices only have a

finite amount of processing power. The full impact of choosing a particular

thread priority is often only observed when the system is under stress, causing

some tasks to succeed at the expense of others. This can only be discovered

through appropriate stress testing. </p> </li>

<li id="GUID-324C10ED-218D-51C9-88E5-C23913C54D0A"><p>When testing new software

use realistic scenarios. Run applications which are likely to be used at the

same time as the new code, for example making and receiving calls at the same

time as sending a text message. Adjust thread priorities as necessary to ensure

a good end-user experience. Choosing too high a priority for your software

can adversely effect the end-user's experience just as severely as using a

priority which is too low. </p> </li>

</ul> </section>

<section id="GUID-357FC034-EEB4-5D9F-B51A-E6A1400247AD"><title>Long running

background tasks</title> <p>Long running tasks usually run in the background.

They use a low priority thread and do not directly respond to user interaction.

They generally use all the CPU time available to them and run for a period

of a fraction of a second to a few seconds. Occasionally long running tasks,

critical to the currently selected UI application, run at a higher priority

for a few seconds. </p> </section>

<section id="GUID-86CD6AE8-33E2-549D-8B51-DE705CE017E7"><title>Special timing

requirements</title> <p>Tasks with special timing requirements fall into two

distinct categories: </p> <ul>

<li id="GUID-DD200274-40B1-5009-8BC9-86C35565B423"><p>Short term, time critical

tasks. </p> <p>These use high priority or real-time threads. </p> </li>

<li id="GUID-26F57B7B-F75E-5E8E-A40F-8A97BF08E1B2"><p>Long running background

tasks such as garbage collection and other cleanup tasks. </p> <p>These use

low priority threads. </p> </li>

</ul> </section>

</conbody></concept>