Thread -Priority Scheme

At any given time, the thread running on Symbian platform is the one with -the highest priority that is ready to run. The priority of a thread is simply -a number; the value determines the priority; the greater the number, the higher -the priority. We call this the true or absolute priority of -the thread.

The Kernel schedules a fixed amount of time called a quantum for -a thread to run on the CPU, and the scheduler chooses the highest priority -thread that is ready to run. Threads of equal priority are executed on a round -robin basis.

The true priority values range from 0 (lowest) to 63 (highest).

The priority range divides into four broad categories:

- - - - -

This is reserved for the null thread, which puts the processor -into idle mode to save power when no other threads are ready to run.

- - -

1 - 23

Used by kernel side threads and user-side applications and servers

- - -

24 -31

Used by kernel side threads and protected system servers, i.e. servers -with the ProtServ capability.

- - -

32 - 63

Reserved for real-time threads running on the kernel side.

- - - -

Priority scheme for general user-side threads
Priority scheme for real-time user-side threads
Thread priority value mapping table
Platform security notes

Priority scheme -for general user-side threads

User-side threads do not allocate true priority -values directly. Instead, they allocate priorities using symbolic values defined -by enums. Symbian platform maps these values to the true value.

There -are two priority allocation schemes:

Process-relative scheme
Process-independent scheme for general user threads

Process-relative scheme

This -scheme bases the true priority of a thread on the priority of its owning -process and the priority of the thread relative to the process. This means -that changing the process priority results in a change to the true priority -of the thread.

A process can be assigned one of eight discrete priorities -represented by the individual enumerators of TProcessPriority, -and range from EPriorityLow, the lowest, to EPrioritySupervisor, -the highest.

In practice, user processes can only be assigned -priorities that are one of the values:

EPriorityLow
EPriorityBackground
EPriorityForeGround
EPriorityHigh

There are a further four process priorities that a user process is -not permitted to set: EPriorityWindowServer, EPriorityFileServer, EPriorityRealTimeServer and EPrioritySupervisor.

A process priority can be assigned:

when the associated .exe is -built; this is done by specifying the value in the .mmp file -that defines the project.
by calling RProcess::SetPriority() and -passing one of the TProcessPriority enum values.

See also : Using -Makmake in the Build -Tools Guide.

The priority of a thread relative to a -process is assigned by calling RThread::SetPriority() and -passing one of the five TThreadPriority enum values:

EPriorityMuchLess
EPriorityLess
EPriorityNormal
EPriorityMore
EPriorityMuchMore

The thread -priority value mapping table shows the true priority of a thread -based on the combination of process priority and process-relative thread priority

Process-independent scheme -for general user threads

In this scheme, the true priority -of a thread is independent of the priority of its owning process. Changing -the priority of the underlying process has no effect on the true priority -of the thread.

The priority of a thread is assigned by calling RThread::SetPriority() and -passing one of the TThreadPriority enum values listed below. -Note that the set of enum values splits into two logical groupings based on -their equivalence to process-relative values. See the thread priority value mapping table.

- - - -

Group 1

Group 2

- - -

EPriorityAbsoluteVeryLow
EPriorityAbsoluteLow
EPriorityAbsoluteBackground
EPriorityAbsoluteForeground
EPriorityAbsoluteHigh

EPriorityAbsoluteLowNormal
EPriorityAbsoluteBackgroundNormal
EPriorityAbsoluteForegroundNormal
EPriorityAbsoluteHighNormal

- - - -

The thread -priority value mapping table shows the resulting true priority -of the thread.

Priority scheme -for real-time user-side threads

This scheme is the same as the process-independent -scheme for general user threads, but with one essential difference -- the range of TThreadPriority enum values to be passed -to RThread::SetPriority() is as shown below. These priorities -map to the true priority values in the range 24 -31, and can only be -set by executables having the ProtServ capability. This range -is referred to as the real time range.

EPriorityAbsoluteRealTime1
EPriorityAbsoluteRealTime2
EPriorityAbsoluteRealTime3
EPriorityAbsoluteRealTime4
EPriorityAbsoluteRealTime5
EPriorityAbsoluteRealTime6
EPriorityAbsoluteRealTime7
EPriorityAbsoluteRealTime8

The thread -priority value mapping table shows the resulting true priority -of the thread.

Thread priority -value mapping table

This table shows the effect of setting priorities, -and the resulting true priority values. You need to be aware that this -may change in the future, and you should never synchronise threads on the -basis of thread priorities. If you need to synchronise threads, use mutexes -or semaphores.

Notes

True priority values -in the shaded region can only be accessed by threads running in processes -with the ProtServ capability.
The process-priority -values : EPriorityWindowServer, EPriorityFileServer and EPrioritySupervisor all -map to the same range of priorities. Along with EPriorityRealTimeServer, -these have historically been used for system servers and other processes needing -access to high priorities suitable for real-time tasks, and their use requires ProtServ capability.
Note that ProtServ capability -will not be granted to general applications for the purpose of gaining access -to the very high thread priorities. This risks breaking important system functionality.

- -

Note that we have used E' as an abbreviation for EPriority in -this diagram.

Platform security -notes

Platform security restrictions prevent thread and process -priorities from being modified by another user process. There is one exception: -by default, applications have “Priority Control” enabled for them which allows -the window server to switch them between foreground and background process -priorities depending on which has foreground focus.

+ + + + + +Thread +Priority SchemeExplains the factors involved in allocating thread priority. +

At any given time, the thread running on Symbian platform is the one with +the highest priority that is ready to run. The priority of a thread is simply +a number; the value determines the priority; the greater the number, the higher +the priority. We call this the true or absolute priority of +the thread.

The Kernel schedules a fixed amount of time called a quantum for +a thread to run on the CPU, and the scheduler chooses the highest priority +thread that is ready to run. Threads of equal priority are executed on a round +robin basis.

The true priority values range from 0 (lowest) to 63 (highest).

The priority range divides into four broad categories:

+ + + + +

This is reserved for the null thread, which puts the processor +into idle mode to save power when no other threads are ready to run.

+ + +

1 - 23

Used by kernel side threads and user-side applications and servers

+ + +

24 -31

Used by kernel side threads and protected system servers, i.e. servers +with the ProtServ capability.

+ + +

32 - 63

Reserved for real-time threads running on the kernel side.

+ + + +

Priority scheme for general user-side threads
Priority scheme for real-time user-side threads
Thread priority value mapping table
Platform security notes

Priority scheme +for general user-side threads

User-side threads do not allocate true priority +values directly. Instead, they allocate priorities using symbolic values defined +by enums. Symbian platform maps these values to the true value.

There +are two priority allocation schemes:

Process-relative scheme
Process-independent scheme for general user threads

Process-relative scheme

This +scheme bases the true priority of a thread on the priority of its owning +process and the priority of the thread relative to the process. This means +that changing the process priority results in a change to the true priority +of the thread.

A process can be assigned one of eight discrete priorities +represented by the individual enumerators of TProcessPriority, +and range from EPriorityLow, the lowest, to EPrioritySupervisor, +the highest.

In practice, user processes can only be assigned +priorities that are one of the values:

EPriorityLow
EPriorityBackground
EPriorityForeGround
EPriorityHigh

There are a further four process priorities that a user process is +not permitted to set: EPriorityWindowServer, EPriorityFileServer, EPriorityRealTimeServer and EPrioritySupervisor.

A process priority can be assigned:

when the associated .exe is +built; this is done by specifying the value in the .mmp file +that defines the project.
by calling RProcess::SetPriority() and +passing one of the TProcessPriority enum values.

See also : Using +Makmake in the Build +Tools Guide.

The priority of a thread relative to a +process is assigned by calling RThread::SetPriority() and +passing one of the five TThreadPriority enum values:

EPriorityMuchLess
EPriorityLess
EPriorityNormal
EPriorityMore
EPriorityMuchMore

The thread +priority value mapping table shows the true priority of a thread +based on the combination of process priority and process-relative thread priority

Process-independent scheme +for general user threads

In this scheme, the true priority +of a thread is independent of the priority of its owning process. Changing +the priority of the underlying process has no effect on the true priority +of the thread.

The priority of a thread is assigned by calling RThread::SetPriority() and +passing one of the TThreadPriority enum values listed below. +Note that the set of enum values splits into two logical groupings based on +their equivalence to process-relative values. See the thread priority value mapping table.

+ + + +

Group 1

Group 2

+ + +

EPriorityAbsoluteVeryLow
EPriorityAbsoluteLow
EPriorityAbsoluteBackground
EPriorityAbsoluteForeground
EPriorityAbsoluteHigh

EPriorityAbsoluteLowNormal
EPriorityAbsoluteBackgroundNormal
EPriorityAbsoluteForegroundNormal
EPriorityAbsoluteHighNormal

+ + + +

The thread +priority value mapping table shows the resulting true priority +of the thread.

Priority scheme +for real-time user-side threads

This scheme is the same as the process-independent +scheme for general user threads, but with one essential difference +- the range of TThreadPriority enum values to be passed +to RThread::SetPriority() is as shown below. These priorities +map to the true priority values in the range 24 -31, and can only be +set by executables having the ProtServ capability. This range +is referred to as the real time range.

EPriorityAbsoluteRealTime1
EPriorityAbsoluteRealTime2
EPriorityAbsoluteRealTime3
EPriorityAbsoluteRealTime4
EPriorityAbsoluteRealTime5
EPriorityAbsoluteRealTime6
EPriorityAbsoluteRealTime7
EPriorityAbsoluteRealTime8

The thread +priority value mapping table shows the resulting true priority +of the thread.

Thread priority +value mapping table

This table shows the effect of setting priorities, +and the resulting true priority values. You need to be aware that this +may change in the future, and you should never synchronise threads on the +basis of thread priorities. If you need to synchronise threads, use mutexes +or semaphores.

Notes

True priority values +in the shaded region can only be accessed by threads running in processes +with the ProtServ capability.
The process-priority +values : EPriorityWindowServer, EPriorityFileServer and EPrioritySupervisor all +map to the same range of priorities. Along with EPriorityRealTimeServer, +these have historically been used for system servers and other processes needing +access to high priorities suitable for real-time tasks, and their use requires ProtServ capability.
Note that ProtServ capability +will not be granted to general applications for the purpose of gaining access +to the very high thread priorities. This risks breaking important system functionality.

+ +

Note that we have used E' as an abbreviation for EPriority in +this diagram.

Platform security +notes

Platform security restrictions prevent thread and process +priorities from being modified by another user process. There is one exception: +by default, applications have “Priority Control” enabled for them which allows +the window server to switch them between foreground and background process +priorities depending on which has foreground focus.

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