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| <h3 class="section">22.3 Process CPU Priority And Scheduling</h3> |
| |
| <p><a name="index-process-priority-2735"></a><a name="index-cpu-priority-2736"></a><a name="index-priority-of-a-process-2737"></a> |
| When multiple processes simultaneously require CPU time, the system's |
| scheduling policy and process CPU priorities determine which processes |
| get it. This section describes how that determination is made and |
| GNU C library functions to control it. |
| |
| <p>It is common to refer to CPU scheduling simply as scheduling and a |
| process' CPU priority simply as the process' priority, with the CPU |
| resource being implied. Bear in mind, though, that CPU time is not the |
| only resource a process uses or that processes contend for. In some |
| cases, it is not even particularly important. Giving a process a high |
| “priority” may have very little effect on how fast a process runs with |
| respect to other processes. The priorities discussed in this section |
| apply only to CPU time. |
| |
| <p>CPU scheduling is a complex issue and different systems do it in wildly |
| different ways. New ideas continually develop and find their way into |
| the intricacies of the various systems' scheduling algorithms. This |
| section discusses the general concepts, some specifics of systems |
| that commonly use the GNU C library, and some standards. |
| |
| <p>For simplicity, we talk about CPU contention as if there is only one CPU |
| in the system. But all the same principles apply when a processor has |
| multiple CPUs, and knowing that the number of processes that can run at |
| any one time is equal to the number of CPUs, you can easily extrapolate |
| the information. |
| |
| <p>The functions described in this section are all defined by the POSIX.1 |
| and POSIX.1b standards (the <code>sched...</code> functions are POSIX.1b). |
| However, POSIX does not define any semantics for the values that these |
| functions get and set. In this chapter, the semantics are based on the |
| Linux kernel's implementation of the POSIX standard. As you will see, |
| the Linux implementation is quite the inverse of what the authors of the |
| POSIX syntax had in mind. |
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| <li><a accesskey="1" href="Absolute-Priority.html#Absolute-Priority">Absolute Priority</a>: The first tier of priority. Posix |
| <li><a accesskey="2" href="Realtime-Scheduling.html#Realtime-Scheduling">Realtime Scheduling</a>: Scheduling among the process nobility |
| <li><a accesskey="3" href="Basic-Scheduling-Functions.html#Basic-Scheduling-Functions">Basic Scheduling Functions</a>: Get/set scheduling policy, priority |
| <li><a accesskey="4" href="Traditional-Scheduling.html#Traditional-Scheduling">Traditional Scheduling</a>: Scheduling among the vulgar masses |
| <li><a accesskey="5" href="CPU-Affinity.html#CPU-Affinity">CPU Affinity</a>: Limiting execution to certain CPUs |
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