I was reading Windows via C/C++. In this book, the author has said:
The system determines the thread's priority level by combining a thread's relative priority with the priority class of the thread's process. This is sometimes referred to as the thread's base priority level. Occasionally, the system boosts the priority level of a thread—usually in response to some I/O event such as a window message or a disk read.
For example, a thread with a normal thread priority in a high-priority class process has a base priority level of 13. If the user presses a key, the system places a WM_KEYDOWN message in the thread's queue. Because a message has appeared in the thread's queue, the thread is schedulable. In addition, the keyboard device driver can tell the system to temporarily boost the thread's level. So the thread might be boosted by 2 and have a current priority level of 15.
So my question is, why we need this mechanism in Windows OS? What will happen if this mechanism has gone?