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more strongly Many device drivers, interrupts, and kernel services, like the memory manager, run in this range And, as you might imagine, given the naming, any delays can cause serious trouble, possibly even data cor ruption if system services cannot respond to requests within a certain window of time Most programs and threads should use the default prior ity level (Normal / Normal) and leave it to the thread scheduler to ensure they are given a fair chance to execute
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Quantums A quantum is the amount of time a thread is permitted to run before possibly being preempted so that the scheduler can run another runnable thread on the processor The specific interval used for thread quantums varies between machines, server, and client OSs and can be modified through configuration Quantums are based on the system clock interval that, on most modern sys tems, ranges from 10 milliseconds to 15 milliseconds per interval The default quantum time on Windows client OSs (eg, Windows 2000, XP, and Vista) is 2 clock intervals The default time on server OSs (eg, Windows Server 2000, Server 2003, and Server 2008) is 1 2 clock intervals Client quantums are shorter than server quantums to increase responsiveness and provide fairer scheduling of threads on the system Contrast this with a server program in which throughput and performance are usually of more importance, where shorter quantums usually mean more context switching and worse per formance You can explicitly select the default client or server settings on any SKU by going to the Advanced settings tab in your Computer 's System Proper ties configuration Select Performance Settings and choose Advanced You will see a dialog that says "Adjust for best performance of" with two options: either "Programs" or "Applications" (depending on the specific OS), which selects the client settings, or "Background services," which selects the server settings There is also a system registry key, \ H K LM\SYS
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TEM\C u r rentCont rolSet\Cont ro l \ P r io r ityCont rol \Wi n 3 2 P r i o r itySepa r a t i o n, which enables you to tune the quantum settings even more A detailed discussion of this capability is not included in this book; please refer to Further Reading, Windows XP Embedded Team, for details Quantum accounting is done inside of an interrupt routine in the OS When this interrupt fires, the actively running thread's quantum counter
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i s decremented; i f the quantum expired, a context switch i s triggered, which may result in a new thread preempting the current one If the quantum has not been exhausted, the thread remains running Note that when a thread voluntarily blocks, its quantum remains intact So if a thread has nearly exhausted its quantum and blocks, for instance, then when its wait is satisfied it may not run for a full quantum Modifications to the thread scheduler 's quantum accounting algorithm were made in Windows Vista and Server 2008 Two problems existed on previous versions of Windows that could lead to unfairness and unpre dictability in the way that thread execution times were measured The first is that interrupts that executed in the context of a thread would count towards that thread's quantum Say that a thread's quantum was 1 5 mil liseconds and 5 milliseconds of that time were spent executing interrupts; in this case, the thread would only be running its code for 1 0 milliseconds Vista no longer accounts for interrupt time when deciding whether to switch out a thread The second problem was that the scheduler didn' t account for threads being scheduled in the middle of a quantum interval The OS uses a timer interrupt routine to account for execution time If this timer was set to execute every 15 milliseconds and some thread was sched uled in the middle of such an interval, say after 5 milliseconds, then when the timer fired next the OS would charge the thread for the full 1 5 mil liseconds, when in fact it only ran for 1 0 milliseconds Vista prefers to undercharge threads instead This same thread would run for nearly a full timer interval longer than it should-since the granularity of the timer routine remains the same-but ensures threads are not unfairly starved
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Priority and Quantum Adjustments A thread's priority or quantum will receive special treatment by the Win dows thread scheduler under some circumstances This includes tempo rary boosts due to various events of interest-such as a CUI thread receiving a new message, starvation detected by the scheduler, etc-or due to the new multimedia class scheduler that Windows provides as of Vista
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