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| <h4 class="subsection">22.3.1 Absolute Priority</h4> |
| |
| <p><a name="index-absolute-priority-2738"></a><a name="index-priority_002c-absolute-2739"></a> |
| Every process has an absolute priority, and it is represented by a number. |
| The higher the number, the higher the absolute priority. |
| |
| <p><a name="index-realtime-CPU-scheduling-2740"></a>On systems of the past, and most systems today, all processes have |
| absolute priority 0 and this section is irrelevant. In that case, |
| See <a href="Traditional-Scheduling.html#Traditional-Scheduling">Traditional Scheduling</a>. Absolute priorities were invented to |
| accommodate realtime systems, in which it is vital that certain processes |
| be able to respond to external events happening in real time, which |
| means they cannot wait around while some other process that <em>wants |
| to</em>, but doesn't <em>need to</em> run occupies the CPU. |
| |
| <p><a name="index-ready-to-run-2741"></a><a name="index-preemptive-scheduling-2742"></a>When two processes are in contention to use the CPU at any instant, the |
| one with the higher absolute priority always gets it. This is true even if the |
| process with the lower priority is already using the CPU (i.e., the |
| scheduling is preemptive). Of course, we're only talking about |
| processes that are running or “ready to run,” which means they are |
| ready to execute instructions right now. When a process blocks to wait |
| for something like I/O, its absolute priority is irrelevant. |
| |
| <p><a name="index-runnable-process-2743"></a><strong>NB:</strong> The term “runnable” is a synonym for “ready to run.” |
| |
| <p>When two processes are running or ready to run and both have the same |
| absolute priority, it's more interesting. In that case, who gets the |
| CPU is determined by the scheduling policy. If the processes have |
| absolute priority 0, the traditional scheduling policy described in |
| <a href="Traditional-Scheduling.html#Traditional-Scheduling">Traditional Scheduling</a> applies. Otherwise, the policies described |
| in <a href="Realtime-Scheduling.html#Realtime-Scheduling">Realtime Scheduling</a> apply. |
| |
| <p>You normally give an absolute priority above 0 only to a process that |
| can be trusted not to hog the CPU. Such processes are designed to block |
| (or terminate) after relatively short CPU runs. |
| |
| <p>A process begins life with the same absolute priority 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 change a process' absolute priority to |
| something other than <code>0</code>. Only a privileged process or the |
| target process' owner can change its absolute priority at all. |
| |
| <p>POSIX requires absolute priority values used with the realtime |
| scheduling policies to be consecutive with a range of at least 32. On |
| Linux, they are 1 through 99. The functions |
| <code>sched_get_priority_max</code> and <code>sched_set_priority_min</code> portably |
| tell you what the range is on a particular system. |
| |
| <h5 class="subsubsection">22.3.1.1 Using Absolute Priority</h5> |
| |
| <p>One thing you must keep in mind when designing real time applications is |
| that having higher absolute priority than any other process doesn't |
| guarantee the process can run continuously. Two things that can wreck a |
| good CPU run are interrupts and page faults. |
| |
| <p>Interrupt handlers live in that limbo between processes. The CPU is |
| executing instructions, but they aren't part of any process. An |
| interrupt will stop even the highest priority process. So you must |
| allow for slight delays and make sure that no device in the system has |
| an interrupt handler that could cause too long a delay between |
| instructions for your process. |
| |
| <p>Similarly, a page fault causes what looks like a straightforward |
| sequence of instructions to take a long time. The fact that other |
| processes get to run while the page faults in is of no consequence, |
| because as soon as the I/O is complete, the high priority process will |
| kick them out and run again, but the wait for the I/O itself could be a |
| problem. To neutralize this threat, use <code>mlock</code> or |
| <code>mlockall</code>. |
| |
| <p>There are a few ramifications of the absoluteness of this priority on a |
| single-CPU system that you need to keep in mind when you choose to set a |
| priority and also when you're working on a program that runs with high |
| absolute priority. Consider a process that has higher absolute priority |
| than any other process in the system and due to a bug in its program, it |
| gets into an infinite loop. It will never cede the CPU. You can't run |
| a command to kill it because your command would need to get the CPU in |
| order to run. The errant program is in complete control. It controls |
| the vertical, it controls the horizontal. |
| |
| <p>There are two ways to avoid this: 1) keep a shell running somewhere with |
| a higher absolute priority. 2) keep a controlling terminal attached to |
| the high priority process group. All the priority in the world won't |
| stop an interrupt handler from running and delivering a signal to the |
| process if you hit Control-C. |
| |
| <p>Some systems use absolute priority as a means of allocating a fixed |
| percentage of CPU time to a process. To do this, a super high priority |
| privileged process constantly monitors the process' CPU usage and raises |
| its absolute priority when the process isn't getting its entitled share |
| and lowers it when the process is exceeding it. |
| |
| <p><strong>NB:</strong> The absolute priority is sometimes called the “static |
| priority.” We don't use that term in this manual because it misses the |
| most important feature of the absolute priority: its absoluteness. |
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