Page 362 - DCAP103_Principle of operating system

Unit 12: Processes and Threads in Windows

Notes
Figure 12.5: Windows 2000 Supports 32 Priorities for Threads

Over the course of time, some patches were made to the basic scheduling algorithm to improve
system performance. Under certain specific conditions, the current priority of a user thread can
be raised above the base priority (by the operating system), but never above priority 15. Since
the array of Figure 12.5 is based on the current priority, changing this priority affects scheduling.
No adjustments are ever made to threads running at priority 15 or higher.

Let us now see when a thread’s priority is raised. First, when an I/O operation completes and
releases a waiting thread, the priority is boosted to give it a chance to run again quickly and start
more I/O. The idea here is to keep the I/O devices busy. The amount of boost depends on the
I/O device, typically 1 for a disk, 2 for a serial line, 6 for the keyboard, and 8 for the sound card.
Second, if a thread was waiting on a semaphore, mutex, or other event, when it is released, it
gets boosted by 2 units if it is in the foreground process (the process controlling the window to
which keyboard input is sent) and 1 unit otherwise. This fix tends to raise interactive processes
above the big crowd at level 8. Finally, if a GUI thread wakes up because window input is now
available, it gets a boost for the same reason.
These boosts are not forever. They take effect immediately, but if a thread uses all of its next
quantum, it loses one point and moves down one queue in the priority array. If it uses up another
full quantum, it moves down to another level, and so on until it hits its base level, where it remains
until it is boosted again. Clearly, if a thread wants good service, it should play a lot of music.
There is an other case in which the system fiddles with the priorities. Imagine that two threads
are working together on a producer-consumer type problem. The producer’s work is harder, so
it gets a high priority, say 12, compared to the consumer’s 4. At a certain point, the producer
has filled up a shared buffer and blocks on a semaphore, as illustrated in Figure 12.6 (a).

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