Windows Thread Synchronization 46

 

 

 

 

Thread Scheduling and Prioritizing

 

This section looks at how you can change the scheduling priority of a thread in UNIX and Windows. Ideally, you want to map Windows priority classes to UNIX scheduling policies and Windows thread priority levels to UNIX priority levels, but unfortunately, it is not this simple. The priority level of a Windows thread is determined by both the priority class of its process and its priority level. The priority class and priority level are combined to form the base priority of each thread. Every thread in Windows has a base priority level determined by the priority value of the thread and the priority class of its owning process. The operating system uses the base priority level of all executable threads to determine which thread gets the next slice of CPU time. Threads are scheduled in a round-robin fashion at each priority level, and scheduling of threads at a lower level will only take place when there are no executable threads at a higher level. UNIX offers both round-robin and FIFO scheduling algorithms, whereas Windows uses only round-robin. This does not mean that Windows is less flexible; it just means that any fine-tuning that was performed on thread scheduling in UNIX has to be implemented differently when using Windows. Table Z lists the base priority levels for combinations of priority class and priority value for Windows.

 

Table Z: Process and Thread Priority for Windows

 

 

Managing Thread Priorities in Windows

 

The Windows API provides a number of functions for managing thread priorities, including the following:

 

1.   GetThreadContext(). This function returns the execution context of the specified thread. The following is an example showing the thread context:

 

CONTEXT context;

WCHAR szBuffer[128];

 

Context.ContextFlags = CONTEXT_FULL | CONTEXT_DEBUG_REGISTERS;

GetThreadContext(GetCurrentThread(),&context);

wprintf(LCS=%X, EIP=%X, FLAGS=%X, DR1=%X\n,context.SegCs, context.Eip, context.EFlags,context.Dr1);

 

2.   GetThreadPriority(). This function returns the assigned thread priority level for the specified thread. To see how thread priority affects the system, a simple test, such as the one that follows, could be added to a simple Windows application:

 

SetThreadPriority(GetCurrentThread(),

THREAD_PRIORITY_LOWEST);

DWORD dwTicks = GetTickCount();

long i, j;

 

for(i = 0; i < 200000; i ++)

for(j = 0; j < 2000; j ++)

wprintf(LTest time=%ld\n, GetTickCount() – dwTicks);

 

Adjusting the thread priority should yield different time deltas.

 

3.   GetThreadPriorityBoost(). This function retrieves the priority boost control state of the specified thread. Threads have dynamic priority, which is the priority that the scheduler uses to identify which thread will execute. Initially, the priority of a thread is the same as its base priority, but the system may increase or decrease the priority to maintain thread responsiveness. Only threads with a priority between 0 and 15 are eligible for dynamic priority boost. The system boosts the dynamic priority of a thread to enhance its responsiveness as follows:

 

1. When a process that uses NORMAL_PRIORITY_CLASS is brought to the foreground, the scheduler boosts the priority class of the process associated with the foreground window so that it is equal to or greater than the priority class of any background processes. The priority class returns to its original setting when the process is no longer in the foreground. In Microsoft Windows, the user can control the boosting of processes that use NORMAL_PRIORITY_CLASS through Control Panel.
2. When a window receives input, such as timer messages, mouse messages, or keyboard input, the scheduler boosts the priority of the thread that owns the window.
3. When the wait conditions for a blocked thread are satisfied, the scheduler boosts the priority of the thread. For example, when a wait operation associated with disk or keyboard I/O finishes, the thread receives a priority boost.

 

4.   SetThreadIdealProcessor(). This function specifies the preferred processor for a specific thread. The system schedules threads on the preferred processor when possible.

5.   SetThreadPriority(). This function changes the priority level for a thread.

6.   SetThreadPriorityBoost(). This function enables or disables dynamic priority boosts by the system.

 

 

 

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