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.

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 +

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:

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:

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.

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.

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.

@@ -125,17 +120,14 @@ -

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.

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
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 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.

EPriorityWindowServer

EPriorityFileServer

EPrioritySupervisor

EPriorityRealTimeServer

ProtServ

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.

+ +

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