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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 943
912
CASE STUDY 2: WINDOWS 8
CHAP. 11
model that UNIX has.
Each of these threads is allocated its own stack and its own
memory to save its registers when not running.
The two threads appear to be a sin-
gle thread because they do not run at the same time.
The user thread operates as an
extension of the kernel thread, running only when the kernel thread switches to it
by returning from kernel mode to user mode.
When a user thread wants to perform
a system call, encounters a page fault, or is preempted, the system enters kernel
mode and switches back to the corresponding kernel thread.
It is normally not pos-
sible to switch between user threads without first switching to the corresponding
kernel thread, switching to the new kernel thread, and then switching to its user
thread.
Most of the time the difference between user and kernel threads is transparent
to the programmer.
However, in Windows 7 Microsoft added a facility called
UMS
(
User-Mode Scheduling
), which exposes the distinction.
UMS is similar to
facilities used in other operating systems, such as
scheduler activations
.
It can be
used to switch between user threads without first having to enter the kernel, provid-
ing the benefits of fibers, but with much better integration into Win32—since it
uses real Win32 threads.
The implementation of UMS has three key elements:
1.
User-mode switching
: a user-mode scheduler can be written to switch
between user threads without entering the kernel. When a user thread
does enter kernel mode, UMS will find the corresponding kernel
thread and immediately switch to it.
2.
Reentering the user-mode scheduler
: when the execution of a kernel
thread blocks to await the availability of a resource, UMS switches to
a special user thread and executes the user-mode scheduler so that a
different user thread can be scheduled to run on the current processor.
This allows the current process to continue using the current proc-
essor for its full turn rather than having to get in line behind other
processes when one of its threads blocks.
3.
System-call completion
: after a blocked kernel thread eventually is
finished, a notification containing the results of the system calls is
queued for the user-mode scheduler so that it can switch to the corres-
ponding user thread next time it makes a scheduling decision.
UMS does not include a user-mode scheduler as part of Windows. UMS is in-
tended as a low-level facility for use by run-time libraries used by programming-
language and server applications to implement lightweight threading models that
do not conflict with kernel-level thread scheduling.
These run-time libraries will
normally implement a user-mode scheduler best suited to their environment. A
summary of these abstractions is given in Fig. 11-23.
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