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Modern Operating Systems by Herbert Bos and Andrew S. Tanenb...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf
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Modern Operating Systems by Herbert Bos and Andrew...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Modern Operating Systems by Herbert...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Page 957
926
CASE STUDY 2: WINDOWS 8
CHAP. 11
Next thread to run
Priority
System
priorities
User
priorities
Zero page thread
31
24
16
8
1
0
Idle thread
Figure 11-26.
Windows supports 32 priorities for threads.
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 levels if it is in the foreground process (the process
controlling the window to which keyboard input is sent) and 1 level 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, and can cause
rescheduling of the CPU.
But if a thread uses all of its next quantum, it loses one
priority level and moves down one queue in the priority array.
If it uses up another
full quantum, it moves down another level, and so on until it hits its base level,
where it remains until it is boosted again.
There is one other case in which the system fiddles with the priorities.
Imag-
ine 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 Fig. 11-27(a).
Before the consumer gets a chance to run again, an unrelated thread at priority
8 becomes ready and starts running, as shown in Fig. 11-27(b). As long as this
thread wants to run, it will be able to, since it has a higher priority than the consu-
mer, and the producer, though even higher, is blocked. Under these circumstances,
the producer will never get to run again until the priority 8 thread gives up. This
Page 958
SEC. 11.4
PROCESSES AND THREADS IN WINDOWS
927
12
4
8
12
Does a down on the
semaphore and blocks
Semaphone
Semaphone
Blocked
Running
Ready
Waiting on the semaphore
Would like to do an up
on the semaphore but
never gets scheduled
(a)
(b)
Figure 11-27.
An example of priority inversion.
problem is well known under the name
priority inversion
.
Windows addresses
priority inversion between kernel threads through a facility in the thread scheduler
called Autoboost.
Autoboost automatically tracks resource dependencies between
threads and boosts the scheduling priority of threads that hold resources needed by
higher-priority threads.
Windows runs on PCs, which usually have only a single interactive session ac-
tive at a time. However, Windows also supports a
terminal server
mode which
supports multiple interactive sessions over the network using
RDP
(
Remote Desk-
top Protocol
). When running multiple user sessions, it is easy for one user to in-
terfere with another by consuming too much processor resources.
Windows imple-
ments a fair-share algorithm,
DFSS
(
Dynamic Fair-Share Scheduling
), which
keeps sessions from running excessively.
DFSS uses
scheduling groups
to
organize the threads in each session.
Within each group the threads are scheduled
according to normal Windows scheduling policies, but each group is given more or
less access to the processors based on how much the group has been running in
aggregate. The relative priorities of the groups are adjusted slowly to allow ignore
short bursts of activity and reduce the amount a group is allowed to run only if it
uses excessive processor time over long periods.
11.5 MEMORY MANAGEMENT
Windows has an extremely sophisticated and complex virtual memory system.
It has a number of Win32 functions for using it, implemented by the memory man-
ager—the largest component of the NTOS executive layer.
In the following sec-
tions we will look at the fundamental concepts, the Win32 API calls, and finally
the implementation.
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