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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 942
SEC. 11.4
are completely unaware of fibers, and applications that attempt to use fibers as if
they were threads will encounter various failures. The kernel has no knowledge of
fibers, and when a fiber enters the kernel, the thread it is executing on may block
and the kernel will schedule an arbitrary thread on the processor, making it
unavailable to run other fibers. For these reasons fibers are rarely used except
when porting code from other systems that explicitly need the functionality pro-
vided by fibers.
Thread Pools and User-Mode Scheduling
The Win32
thread pool
is a facility that builds on top of the Windows thread
model to provide a better abstraction for certain types of programs.
Thread crea-
tion is too expensive to be invoked every time a program wants to execute a small
task concurrently with other tasks in order to take advantage of multiple proc-
essors. Tasks can be grouped together into larger tasks but this reduces the amount
of exploitable concurrency in the program.
An alternative approach is for a pro-
gram to allocate a limited number of threads, and maintain a queue of tasks that
need to be run.
As a thread finishes the execution of a task, it takes another one
from the queue.
This model separates the resource-management issues (how many
processors are available and how many threads should be created) from the pro-
gramming model (what is a task and how are tasks synchronized).
Windows for-
malizes this solution into the Win32 thread pool, a set of APIs for automatically
managing a dynamic pool of threads and dispatching tasks to them.
Thread pools are not a perfect solution, because when a thread blocks for some
resource in the middle of a task, the thread cannot switch to a different task.
the thread pool will inevitably create more threads than there are processors avail-
able, so if runnable threads are available to be scheduled even when other threads
have blocked. The thread pool is integrated with many of the common synchroni-
zation mechanisms, such as awaiting the completion of I/O or blocking until a ker-
nel event is signaled.
Synchronization can be used as triggers for queuing a task so
threads are not assigned the task before it is ready to run.
The implementation of the thread pool uses the same queue facility provided
for synchronization with I/O completion, together with a kernel-mode thread fac-
tory which adds more threads to the process as needed to keep the available num-
ber of processors busy.
Small tasks exist in many applications, but particularly in
those that provide services in the client/server model of computing, where a stream
of requests are sent from the clients to the server.
Use of a thread pool for these
scenarios improves the efficiency of the system by reducing the overhead of creat-
ing threads and moving the decisions about how to manage the threads in the pool
out of the application and into the operating system.
What programmers see as a single Windows thread is actually two threads: one
that runs in kernel mode and one that runs in user mode.
This is precisely the same

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