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p u b l i c c l a s s Thread { p u b l i c ThreadPriority P riority { get ; set ; } }
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(Note that the System D i a g n o st i c s P r o c e s s T h r e a d class also offers a
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P r i o r i t y L e v e l property, which also allows you to adjust a thread's relative
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T h re a d 5 c h ed u l l n ,
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priority Using it, however, is discouraged Setting a managed thread's priority via the T h r e a d class enables the CLR to do additional bookkeeping which is used, for example, to reset priorities if a thread is accidentally returned back to the thread pool with a higher priority than normal) There are seven possible relative priority offsets you may assign to a thread, two of which are not supported in managed code (unless you use P ro c e s sTh read, which supports all seven) Most of these offsets either add or subtract a constant, though two of them effectively set the thread's pri ority level to an absolute value depending on the process priority class They are shown in Table 42
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TABLE 42: Wi ndows relative priorities a n d Win 3 2 and N ET enum va lues Title Win 3 2 Constant Value N ET E n u m Va lue Level Modifier
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Absolute value: 31 for real-time range, 15 for dynamic range +2 +1
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Highest Above Normal Normal Below Normal Lowest Idle
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THREAD_PRIORITV_HIGHEST THREAD_PRIORITV_ABOVE_NORMAL
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Highest AboveNorma l
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-2 Absolute value: 15 for real-time range, 1 for dynamic range
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To take an example, imagine w e have a process with the default priority class of Normal (B) When we create a thread, it will also by default be given the Normal relative priority (+0) Therefore, the thread's level is B If we were to instead assign the thread a different relative pri ority, say, Highest (+2), then this thread would have a level of 10 (B + 2) If, on the other hand, we gave a thread Highest relative priority (+2) inside of a process that has a priority class of High ( 1 3), then the thread's resulting priority level would be 15 ( 1 3 + 2), the highest possible priority level in the dynamic range Notice that the default real-time priority level (24) plus THREAD_PRIOR ITY_H I G H E S T or minus THR EAD_P R I O R I TY_LOW ES T still leaves many levels inaccessible That is, 24 + 2 is 26, yet the maximum in the real-time range and class is 31, and similarly 24 - 2 is 22, yet the minimum is 1 6 This is why Set T h r e a d P r io r ity takes an i n t as its argument To access the other values in the range, you can pass values here by hand: -7, -6, -5, -4, -3, 3, 4, 5, and 6 On Windows Vista and Server 200B, a new feature called I / O Prioriti zation has been added This regulates the scheduling of I / Os because con tention for the disk can artificially boost the priority of lower priority processes and threads by allowing them to interfere with higher priority ones Five priorities are used : Critical, High, Medium, Low, and Very Low Assignment of priority to an I / O request is handled primarily by the OS and drivers, although you have some control over it by assigning thread priorities By default, all I / O under a priority of Medium, but you may pass the value P ROC ESS_MOD E_BAC KG ROUND_B E G I N to Set P r iorityC l a s s to lower the I / O Priority to Very Low, and PROC E S S_MOD E_BAC KG ROUND_END to revert it Similarly, you can pass T H R E AD_MODE_BAC KG ROUND_B E G I N to the SetTh re a d P r i o r ity function to lower I / O Priority for that particular thread, and TH R E AD_MODE_BAC KG ROUND_END to revert this change This is used by programs such as the Windows Search Indexer to prevent it from interfering with other interactive applications Now that we've seen how priority level is calculated and how to adjust priority classes and thread relative priorities, some words of warning are appropriate Any priorities over the Above Normal class should be avoided almost entirely Using them will interfere with other system services that usually run at high priorities within the dynamic range, possibly causing hangs and system instability Using real-time priorities is discouraged even