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<div class="Head1">
<h2>Process and Thread Priorities</h2>
</div>
<div>
<p>
Internally the scheduler always deals with nanokernel threads,
NThread objects, and their associated priority between 0 (lowest)
and 63 (highest). In general, a thread with a higher priority that is ready to
run will always run in preference to threads with a lower priority. The only
exception is where a higher priority thread waits on a nanokernel fast mutex
held by a lower priority thread. In this case, the higher priority thread will
yield to the lower priority thread holding the mutex.
</p>
<p>
A Symbian OS thread, a DThread object, has an embedded
NThread, which enables it to be scheduled by the nanokernel.
</p>
<p>
There are two ways of setting a priority for Symbian OS thread:
</p>
<ul>
<li>
<p>
using the two-level priority scheme
</p>
</li>
<li>
<p>
using an absolute priority.
</p>
</li>
</ul>
</div>
<div class="Head3">
<h3><span class="Bodytext"><a name="1.4.7"></a></span>The two level priority scheme</h3>
</div><div>
<p>
In this scheme, a Symbian OS thread priority is relative to the
priority of its owning process. By default, Symbian OS threads inherit the
priority of their owning process when they are created. This priority can be
raised or lowered relative to the process priority - this just sets the
thread’s priority to the process priority plus or minus a specified priority
weighting. If the priority of the process is changed, the priority of its
threads will change relative to other threads in the system but will remain the
same relative to each other.
</p>
<p>
The default priority of a process is
EPriorityForgeround, which is an absolute priority of 350. Threads
by default are created with relative priority EPriorityNormal
which sets them to the same priority as the owning process. The window server
lowers the priority of background UI processes to
EPriorityBackground (250).
</p>
<p>
The NULL thread, also known as the idle thread, runs at priority 0,
and means that it will only run when there are no other threads ready to run.
</p>
<p>
Symbian OS thread priorities map onto NThread priorities
in the range 1 to 31 as shown in the table below.
</p><table cellpadding="1" border="0" cellspacing="0"><tr><td
class="TableWrap"><table cellpadding="5" cellspacing="1"
border="0"><tr valign="top"><td class="Cell">
<p>
<b>Thread priority</b>
</p></td><td class="Cell">
<p>
Idle
</p></td><td class="Cell">
<p>
Much Less
</p></td><td class="Cell">
<p>
Less
</p></td><td class="Cell">
<p>
Normal
</p></td><td class="Cell">
<p>
More
</p></td><td class="Cell">
<p>
Much More
</p></td><td class="Cell">
<p>
Real Time
</p></td></tr>
<tr valign="top"><td class="Cell">
<p>
<b>Process priority</b>
</p></td><td class="Cell">
<p>
</p></td><td class="Cell">
<p>
</p></td><td class="Cell">
<p>
</p></td><td class="Cell">
<p>
</p></td><td class="Cell">
<p>
</p></td><td class="Cell">
<p>
</p></td><td class="Cell">
<p>
</p></td></tr>
<tr valign="top"><td class="Cell">
<p>
Low
</p></td><td class="Cell">
<p>
1</p></td><td class="Cell">
<p>
1</p></td><td class="Cell">
<p>
2</p></td><td class="Cell">
<p>
3</p></td><td class="Cell">
<p>
4</p></td><td class="Cell">
<p>
5</p></td><td class="Cell">
<p>
22
</p></td></tr>
<tr valign="top"><td class="Cell">
<p>
Background
</p></td><td class="Cell">
<p>
3</p></td><td class="Cell">
<p>
5</p></td><td class="Cell">
<p>
6</p></td><td class="Cell">
<p>
7</p></td><td class="Cell">
<p>
8</p></td><td class="Cell">
<p>
9</p></td><td class="Cell">
<p>
22
</p></td></tr>
<tr valign="top"><td class="Cell">
<p>
Foreground
</p></td><td class="Cell">
<p>
3</p></td><td class="Cell">
<p>
10
</p></td><td class="Cell">
<p>
11
</p></td><td class="Cell">
<p>
12
</p></td><td class="Cell">
<p>
13
</p></td><td class="Cell">
<p>
14
</p></td><td class="Cell">
<p>
22
</p></td></tr>
<tr valign="top"><td class="Cell">
<p>
High
</p></td><td class="Cell">
<p>
3</p></td><td class="Cell">
<p>
17
</p></td><td class="Cell">
<p>
18
</p></td><td class="Cell">
<p>
19
</p></td><td class="Cell">
<p>
20
</p></td><td class="Cell">
<p>
22
</p></td><td class="Cell">
<p>
23
</p></td></tr>
<tr valign="top"><td class="Cell">
<p>
SystemServer1
</p></td><td class="Cell">
<p>
9</p></td><td class="Cell">
<p>
15
</p></td><td class="Cell">
<p>
16
</p></td><td class="Cell">
<p>
21
</p></td><td class="Cell">
<p>
24
</p></td><td class="Cell">
<p>
25
</p></td><td class="Cell">
<p>
28
</p></td></tr>
<tr valign="top"><td class="Cell">
<p>
SystemServer2
</p></td><td class="Cell">
<p>
9</p></td><td class="Cell">
<p>
15
</p></td><td class="Cell">
<p>
16
</p></td><td class="Cell">
<p>
21
</p></td><td class="Cell">
<p>
24
</p></td><td class="Cell">
<p>
25
</p></td><td class="Cell">
<p>
28
</p></td></tr>
<tr valign="top"><td class="Cell">
<p>
SystemServer3
</p></td><td class="Cell">
<p>
9</p></td><td class="Cell">
<p>
15
</p></td><td class="Cell">
<p>
16
</p></td><td class="Cell">
<p>
21
</p></td><td class="Cell">
<p>
24
</p></td><td class="Cell">
<p>
25
</p></td><td class="Cell">
<p>
28
</p></td></tr>
<tr valign="top"><td class="Cell">
<p>
RealTimeServer
</p></td><td class="Cell">
<p>
18
</p></td><td class="Cell">
<p>
26
</p></td><td class="Cell">
<p>
27
</p></td><td class="Cell">
<p>
28
</p></td><td class="Cell">
<p>
29
</p></td><td class="Cell">
<p>
30
</p></td><td class="Cell">
<p>
31
</p></td></tr>
</table></td></tr></table>
<p>
where:
</p>
<ul>
<li>
<p>
the process priority values are defined by the internal Symbian
OS enum TProcPriority, defined in
...\e32\include\kernel\kern_priv.h. The symbols in the table
correspond to the symbols in the enum.
</p>
</li>
<li>
<p>
the thread priority values are defined by the internal Symbian OS
enum TThrdPriority, defined in
...\e32\include\kernel\kern_priv.h. The symbols in the table
correspond to the symbols in the enum.
</p>
</li>
</ul>
</div>
<div class="Head3">
<h3><span class="Bodytext"><a name="1.4.8"></a></span>Absolute priority scheme</h3>
</div><div>
<p>
It is possible to set an absolute priority that is not relative to
the process priority; it is not affected by changes in the process priority.
</p>
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