Categories: Symbian C++ | Symbian OS | S60 | Essential Idioms

This page was last modified 21:32, 13 December 2007.

From Forum Nokia Wiki

On Symbian OS, threads are pre-emptively scheduled and the currently running thread is the highest priority thread ready to run. If there are two or more threads with equal priority, they are time-sliced on a round-robin basis. The priority of a thread is a number: the higher the value, the higher the priority.

When writing multithreaded code, you should consider the relative priorities of your threads carefully. You should not arbitrarily assign a thread a high priority, otherwise it may pre-empt other threads in the system and affect their response times. Even those threads which may legitimately be assigned high priorities must endeavor to make their event-handling service complete rapidly, so they can yield the system and allow other threads to execute.

A thread has an absolute priority which is calculated from the priority assigned to the thread by a call to RThread::SetPriority() and optionally combined with the priority assigned to the process in which it runs. The TThreadPriority and TProcessPriority enumerations, taken from e32std.h, are shown below.

enum TThreadPriority
{
   EPriorityNull=(-30),
   EPriorityMuchLess=(-20),
   EPriorityLess=(-10),
   EPriorityNormal=0,
   EPriorityMore=10,
   EPriorityMuchMore=20,
   EPriorityRealTime=30,
   EPriorityAbsoluteVeryLow=100,
   EPriorityAbsoluteLow=200,
   EPriorityAbsoluteBackground=300,
   EPriorityAbsoluteForeground=400,
   EPriorityAbsoluteHigh=500
};
 
enum TProcessPriority
{
   EPriorityLow=150,
   EPriorityBackground=250,
   EPriorityForeground=350,
   EPriorityHigh=450,
   EPriorityWindowServer=650,
   EPriorityFileServer=750,
   EPriorityRealTimeServer=850,
   EPrioritySupervisor=950
};

All threads are created with priority EPriorityNormal by default. When a thread is created it is initially put into a suspended state and does not begin to run until Resume() is called on its handle. This allows the priority of the thread to be changed by a call to SetPriority() before it starts to run, although the priority of a thread can also be changed at any time.

Links

Inter-Thread Communication

Process and threads, how to find them

Writeable Static Data