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| <h4 class="subsection">22.3.2 Realtime Scheduling</h4> |
| |
| <p><a name="index-realtime-scheduling-2744"></a> |
| Whenever two processes with the same absolute priority are ready to run, |
| the kernel has a decision to make, because only one can run at a time. |
| If the processes have absolute priority 0, the kernel makes this decision |
| as described in <a href="Traditional-Scheduling.html#Traditional-Scheduling">Traditional Scheduling</a>. Otherwise, the decision |
| is as described in this section. |
| |
| <p>If two processes are ready to run but have different absolute priorities, |
| the decision is much simpler, and is described in <a href="Absolute-Priority.html#Absolute-Priority">Absolute Priority</a>. |
| |
| <p>Each process has a scheduling policy. For processes with absolute |
| priority other than zero, there are two available: |
| |
| <ol type=1 start=1> |
| <li>First Come First Served |
| <li>Round Robin |
| </ol> |
| |
| <p>The most sensible case is where all the processes with a certain |
| absolute priority have the same scheduling policy. We'll discuss that |
| first. |
| |
| <p>In Round Robin, processes share the CPU, each one running for a small |
| quantum of time (“time slice”) and then yielding to another in a |
| circular fashion. Of course, only processes that are ready to run and |
| have the same absolute priority are in this circle. |
| |
| <p>In First Come First Served, the process that has been waiting the |
| longest to run gets the CPU, and it keeps it until it voluntarily |
| relinquishes the CPU, runs out of things to do (blocks), or gets |
| preempted by a higher priority process. |
| |
| <p>First Come First Served, along with maximal absolute priority and |
| careful control of interrupts and page faults, is the one to use when a |
| process absolutely, positively has to run at full CPU speed or not at |
| all. |
| |
| <p>Judicious use of <code>sched_yield</code> function invocations by processes |
| with First Come First Served scheduling policy forms a good compromise |
| between Round Robin and First Come First Served. |
| |
| <p>To understand how scheduling works when processes of different scheduling |
| policies occupy the same absolute priority, you have to know the nitty |
| gritty details of how processes enter and exit the ready to run list: |
| |
| <p>In both cases, the ready to run list is organized as a true queue, where |
| a process gets pushed onto the tail when it becomes ready to run and is |
| popped off the head when the scheduler decides to run it. Note that |
| ready to run and running are two mutually exclusive states. When the |
| scheduler runs a process, that process is no longer ready to run and no |
| longer in the ready to run list. When the process stops running, it |
| may go back to being ready to run again. |
| |
| <p>The only difference between a process that is assigned the Round Robin |
| scheduling policy and a process that is assigned First Come First Serve |
| is that in the former case, the process is automatically booted off the |
| CPU after a certain amount of time. When that happens, the process goes |
| back to being ready to run, which means it enters the queue at the tail. |
| The time quantum we're talking about is small. Really small. This is |
| not your father's timesharing. For example, with the Linux kernel, the |
| round robin time slice is a thousand times shorter than its typical |
| time slice for traditional scheduling. |
| |
| <p>A process begins life with the same scheduling policy as its parent process. |
| Functions described in <a href="Basic-Scheduling-Functions.html#Basic-Scheduling-Functions">Basic Scheduling Functions</a> can change it. |
| |
| <p>Only a privileged process can set the scheduling policy of a process |
| that has absolute priority higher than 0. |
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