Each thread has a dynamic priority
. This is the priority the scheduler uses to determine which thread to
execute. Initially, a thread's dynamic priority is the same as its base priority. The
system can boost and lower the dynamic priority, to ensure that it is
responsive and that no threads are starved for processor time. The system does not boost
the priority of threads with a base priority level between 16 and 31. Only
threads with a base priority between 0 and 15 receive dynamic priority boosts.
The system boosts the dynamic priority of a thread to enhance its
responsiveness as follows.
- 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 greater than or equal to the priority class of any
background processes. The priority class returns to its original setting when
the process is no longer in the foreground.
The user can control the boosting of processes that use NORMAL_PRIORITY_CLASS
through the System control panel application.
- 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
- 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.
You can disable the priority-boosting feature by calling the SetProcessPriorityBoost
function. To determine whether this feature has been disabled, call the GetProcessPriorityBoost
After raising a thread's dynamic priority, the scheduler reduces that priority
by one level each time the thread completes a time slice, until the thread
drops back to its base priority. A thread's dynamic priority is never less than
its base priority.
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