Thread scheduling



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Thread Scheduling

we introduced threads to the process model, distinguishing between user-level and kernel-level threads. On operating systems that support them, it is kernel-level threads—not processes—that are being scheduled by the operating system. User-level threads are managed by a thread library, and the kernel is unaware of them. To run on a CPU, user-level threads must ultimately be mapped to an associated kernel-level thread, although this mapping may be indirect and may use a lightweight process (LWP). In this section, we explore scheduling issues involving user-level and kernel-level threads and offer specific examples of scheduling for Pthreads.

Contention Scope

 One distinction between user-level and kernel-level threads lies in how they are scheduled. On systems implementing the many-to-one (Section 4.2.1) and many-to-many (Section 4.2.3) models, the thread library schedules user-level threads to run on an available LWP, a scheme known as process-contention scope (PCS), since competition for the CPU takes place among threads belonging to the same process. When we say the thread library schedules user threads onto available LWPs, we do not mean that the thread is actually running on a CPU; this would require the operating system to schedule the kernel thread onto a physical CPU. To decide which kernel thread to schedule onto a CPU, the kernel uses system-contention scope (SCS).

Competition for the CPU with SCS scheduling takes place among all threads in the system. Systems using the one-to-one model (such as Windows XP, Solaris 9, and Linux) schedule threads using only SCS. Typically, PCS is done according to priority—the scheduler selects the runnable thread with the highest priority to run. User-level thread priorities are set by the programmer and are not adjusted by the thread library, although some thread libraries may allow the programmer to change the priority of a thread. It is important to note that PCS will typically preempt the thread currently running in favor of a higher-priority thread; however, there is no guarantee of time slicing (Section 5.3.4) among threads of equal priority.

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Pthread Scheduling

 We provided a sample POSIX Pthread program in Section 4.3.1, along with an introduction to thread creation with Pthreads. Now, we highlight the POSIX Pthread API that allows specifying either PCS or SCS during thread creation. Pthreads identifies the following contention scope values: ® PTHREAD_SCOPEJPROCESS schedules threads using PCS scheduling.

• PTHREAD-SCOPE_SYSTEM schedules threads using SCS scheduling.

On systems implementing the many-to-many model (Section 4.2.3), the PTHREAD_SCOPE_PROCESS policy schedules user-level threads onto available LVVPs. The number of LWFs is maintained by the thread library, perhaps using scheduler activations (Section 4.4.6). The PT HREAD_SCOPE_SYSTEM scheduling policy will create and bind an LWP for each user-level thread on many-to-many systems, effectively mapping threads using the one-to-one policy (Section 4'.2.2). The Pthread IPC provides the following two functions for getting—and setting—-the contention scope policy:

• pthread_attr_setscope(pthread_attr_t *attr, int scope)

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 • pthread_attr_getscope(pthread_attr_t *attr, int *scope)

The first parameter for both functions contains a pointer to the attri^ite set for the thread. The second parameter for the pthread^attr^setscope 0 function is passed either the PTHREAD.SCOPE..SYSTEM or PTHREAD_5COPE_PROCESS value, indicating how the contention scope is to be set.

In the case of pthread^attr_getscope(), this second parameter contains a pointer to an int value that is set to the current value of the contention scope. If an error occurs, each of these functions returns non-zero values. In Figure 5.9, we illustrate a Pthread program that first determines the existing contention scope and sets it to PTHREAD.SCOPE.PROCESS. It then creates five separate threads that will run using the SCS scheduling policy. Note that on some systems, only certain contention scope values are allowed. For example, Linux and Mac OS X systems allow only PTHREAD_SCOPE_SYSTEM.


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