Windows® Internals, Sixth Edition, Part 1

Timer coalescing works on the assumption that most drivers and user-mode applications do not particularly care about the exact firing period of their timers (except in the case of multimedia applications, for example). This “don’t care” region actually grows as the original timer period grows—an application waking up every 30 seconds probably doesn’t mind waking up every 31 or 29 seconds instead, while a driver polling every second could probably poll every second plus or minus 50 ms without too many problems. The important guarantee most periodic timers depend on is that their firing period remains constant within a certain range—for example, when a timer has been changed to fire every second plus 50 ms, it continues to fire within that range forever, not sometimes at every two seconds and other times at half a second. Even so, not all timers are ready to be coalesced into coarser granularities, so Windows enables this mechanism only for timers that have marked themselves as coalescable, either through the KeSetCoalescableTimer kernel API or through its user-mode counterpart, SetWaitableTimerEx.

With these APIs, driver and application developers are free to provide the kernel with the maximum tolerance (or tolerably delay) that their timer will endure, which is defined as the maximum amount of time past the requested period at which the timer will still function correctly. (In the previous example, the 1-second timer had a tolerance of 50 milliseconds.) The recommended minimum tolerance is 32 ms, which corresponds to about twice the 15.6-ms clock tick—any smaller value wouldn’t really result in any coalescing, because the expiring timer could not be moved even from one clock tick to the next. Regardless of the tolerance that is specified, Windows aligns the timer to one of four preferred coalescing intervals: 1 second, 250 ms, 100 ms, or 50 ms.

When a tolerable delay is set for a periodic timer, Windows uses a process called shifting, which causes the timer to drift between periods until it gets aligned to the most optimal multiple of the period interval within the preferred coalescing interval associated with the specified tolerance (which is then encoded in the dispatcher header). For absolute timers, the list of preferred coalescing intervals is scanned, and a preferred expiration time is generated based on the closest acceptable coalescing interval to the maximum tolerance the caller specified. This behavior means that absolute timers are always pushed out as far as possible past their real expiration point, which spreads out timers as far as possible and creates longer sleep times on the processors.

Now with timer coalescing, refer back to Figure 3-11 and assume all the timers specified tolerances and are thus coalescable. In one scenario, Windows could decide to coalesce the timers as shown in Figure 3-12. Notice that now, processor 1 receives a total of only three clock interrupts, significantly increasing the periods of idle sleep, thus achieving a lower C-state. Furthermore, there is less work to do for some of the clock interrupts on processor 0, possibly removing the latency of requiring a drop to DISPATCH_LEVEL at each clock interrupt.

Figure 3-12. Timer coalescing

Exception Dispatching

In contrast to interrupts, which can occur at any time, exceptions are conditions that result directly from the execution of the program that is running. Windows uses a facility known as structured exception handling, which allows applications to gain control when exceptions occur. The application can then fix the condition and return to the place the exception occurred, unwind the stack (thus terminating execution of the subroutine that raised the exception), or declare back to the system that the exception isn’t recognized and the system should continue searching for an exception handler that might process the exception. This section assumes you’re familiar with the basic concepts behind Windows structured exception handling—if you’re not, you should read the overview in the Windows API reference documentation in the Windows SDK or Chapters 23 through 25 in Jeffrey Richter and Christophe Nasarre’s book Windows via C/C++ (Microsoft Press, 2007) before proceeding. Keep in mind that although exception handling is made accessible through language extensions (for example, the __try construct in Microsoft Visual C++), it is a system mechanism and hence isn’t language specific.