1SCHED_SETSCHEDULER(2) Linux Programmer's Manual SCHED_SETSCHEDULER(2)
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6 sched_setscheduler, sched_getscheduler - set and get scheduling pol‐
7 icy/parameters
8
10 #include <sched.h>
11 int sched_setscheduler(pid_t pid, int policy,
13 const struct sched_param *param);
14 int sched_getscheduler(pid_t pid);
16 struct sched_param {
18 ...
19 int sched_priority;
20 ...
21 };
22
24 sched_setscheduler() sets both the scheduling policy and the associated
25 parameters for the process whose ID is specified in pid. If pid equals
26 zero, the scheduling policy and parameters of the calling process will
27 be set. The interpretation of the argument param depends on the
28 selected policy. Currently, Linux supports the following "normal"
29 (i.e., non-real-time) scheduling policies:
30 SCHED_OTHER the standard round-robin time-sharing policy;
32 SCHED_BATCH for "batch" style execution of processes; and
34 SCHED_IDLE for running very low priority background jobs.
36 The following "real-time" policies are also supported, for special
38 time-critical applications that need precise control over the way in
39 which runnable processes are selected for execution:
40 SCHED_FIFO a first-in, first-out policy; and
42 SCHED_RR a round-robin policy.
44 The semantics of each of these policies are detailed below.
46 sched_getscheduler() queries the scheduling policy currently applied to
48 the process identified by pid. If pid equals zero, the policy of the
49 calling process will be retrieved.
50 Scheduling Policies
52 The scheduler is the kernel component that decides which runnable
53 process will be executed by the CPU next. Each process has an associ‐
54 ated scheduling policy and a static scheduling priority, sched_prior‐
55 ity; these are the settings that are modified by sched_setscheduler().
56 The scheduler makes it decisions based on knowledge of the scheduling
57 policy and static priority of all processes on the system.
58 For processes scheduled under one of the normal scheduling policies
60 (SCHED_OTHER, SCHED_IDLE, SCHED_BATCH), sched_priority is not used in
61 scheduling decisions (it must be specified as 0).
62 Processes scheduled under one of the real-time policies (SCHED_FIFO,
64 SCHED_RR) have a sched_priority value in the range 1 (low) to 99
65 (high). (As the numbers imply, real-time processes always have higher
66 priority than normal processes.) Note well: POSIX.1-2001 only requires
67 an implementation to support a minimum 32 distinct priority levels for
68 the real-time policies, and some systems supply just this minimum.
69 Portable programs should use sched_get_priority_min(2) and
70 sched_get_priority_max(2) to find the range of priorities supported for
71 a particular policy.
72 Conceptually, the scheduler maintains a list of runnable processes for
74 each possible sched_priority value. In order to determine which
75 process runs next, the scheduler looks for the non-empty list with the
76 highest static priority and selects the process at the head of this
77 list.
78 A process's scheduling policy determines where it will be inserted into
80 the list of processes with equal static priority and how it will move
81 inside this list.
82 All scheduling is preemptive: if a process with a higher static prior‐
84 ity becomes ready to run, the currently running process will be pre‐
85 empted and returned to the wait list for its static priority level.
86 The scheduling policy only determines the ordering within the list of
87 runnable processes with equal static priority.
88 SCHED_FIFO: First In-First Out scheduling
90 SCHED_FIFO can only be used with static priorities higher than 0, which
91 means that when a SCHED_FIFO processes becomes runnable, it will always
92 immediately preempt any currently running SCHED_OTHER, SCHED_BATCH, or
93 SCHED_IDLE process. SCHED_FIFO is a simple scheduling algorithm with‐
94 out time slicing. For processes scheduled under the SCHED_FIFO policy,
95 the following rules apply:
96 * A SCHED_FIFO process that has been preempted by another process of
98 higher priority will stay at the head of the list for its priority
99 and will resume execution as soon as all processes of higher prior‐
100 ity are blocked again.
101 * When a SCHED_FIFO process becomes runnable, it will be inserted at
103 the end of the list for its priority.
104 * A call to sched_setscheduler() or sched_setparam(2) will put the
106 SCHED_FIFO (or SCHED_RR) process identified by pid at the start of
107 the list if it was runnable. As a consequence, it may preempt the
108 currently running process if it has the same priority.
109 (POSIX.1-2001 specifies that the process should go to the end of the
110 list.)
111 * A process calling sched_yield(2) will be put at the end of the list.
113 No other events will move a process scheduled under the SCHED_FIFO pol‐
115 icy in the wait list of runnable processes with equal static priority.
116 A SCHED_FIFO process runs until either it is blocked by an I/O request,
118 it is preempted by a higher priority process, or it calls
119 sched_yield(2).
120 SCHED_RR: Round Robin scheduling
122 SCHED_RR is a simple enhancement of SCHED_FIFO. Everything described
123 above for SCHED_FIFO also applies to SCHED_RR, except that each process
124 is only allowed to run for a maximum time quantum. If a SCHED_RR
125 process has been running for a time period equal to or longer than the
126 time quantum, it will be put at the end of the list for its priority.
127 A SCHED_RR process that has been preempted by a higher priority process
128 and subsequently resumes execution as a running process will complete
129 the unexpired portion of its round robin time quantum. The length of
130 the time quantum can be retrieved using sched_rr_get_interval(2).
131 SCHED_OTHER: Default Linux time-sharing scheduling
133 SCHED_OTHER can only be used at static priority 0. SCHED_OTHER is the
134 standard Linux time-sharing scheduler that is intended for all pro‐
135 cesses that do not require the special real-time mechanisms. The
136 process to run is chosen from the static priority 0 list based on a
137 dynamic priority that is determined only inside this list. The dynamic
138 priority is based on the nice value (set by nice(2) or setpriority(2))
139 and increased for each time quantum the process is ready to run, but
140 denied to run by the scheduler. This ensures fair progress among all
141 SCHED_OTHER processes.
142 SCHED_BATCH: Scheduling batch processes
144 (Since Linux 2.6.16.) SCHED_BATCH can only be used at static priority
145 0. This policy is similar to SCHED_OTHER in that it schedules the
146 process according to its dynamic priority (based on the nice value).
147 The difference is that this policy will cause the scheduler to always
148 assume that the process is CPU-intensive. Consequently, the scheduler
149 will apply a small scheduling penalty with respect to wakeup behaviour,
150 so that this process is mildly disfavored in scheduling decisions.
151 This policy is useful for workloads that are non-interactive, but do
153 not want to lower their nice value, and for workloads that want a
154 deterministic scheduling policy without interactivity causing extra
155 preemptions (between the workload's tasks).
156 SCHED_IDLE: Scheduling very low priority jobs
158 (Since Linux 2.6.23.) SCHED_IDLE can only be used at static priority
159 0; the process nice value has no influence for this policy.
160 This policy is intended for running jobs at extremely low priority
162 (lower even than a +19 nice value with the SCHED_OTHER or SCHED_BATCH
163 policies).
164 Privileges and resource limits
166 In Linux kernels before 2.6.12, only privileged (CAP_SYS_NICE) pro‐
167 cesses can set a non-zero static priority (i.e., set a real-time sched‐
168 uling policy). The only change that an unprivileged process can make
169 is to set the SCHED_OTHER policy, and this can only be done if the
170 effective user ID of the caller of sched_setscheduler() matches the
171 real or effective user ID of the target process (i.e., the process
172 specified by pid) whose policy is being changed.
173 Since Linux 2.6.12, the RLIMIT_RTPRIO resource limit defines a ceiling
175 on an unprivileged process's static priority for the SCHED_RR and
176 SCHED_FIFO policies. The rules for changing scheduling policy and pri‐
177 ority are as follows:
178 * If an unprivileged process has a non-zero RLIMIT_RTPRIO soft limit,
180 then it can change its scheduling policy and priority, subject to the
181 restriction that the priority cannot be set to a value higher than
182 the maximum of its current priority and its RLIMIT_RTPRIO soft limit.
183 * If the RLIMIT_RTPRIO soft limit is 0, then the only permitted changes
185 are to lower the priority, or to switch to a non-real-time policy.
186 * Subject to the same rules, another unprivileged process can also make
188 these changes, as long as the effective user ID of the process making
189 the change matches the real or effective user ID of the target
190 process.
191 * Special rules apply for the SCHED_IDLE: an unprivileged process oper‐
193 ating under this policy cannot change its policy, regardless of the
194 value of its RLIMIT_RTPRIO resource limit.
195 Privileged (CAP_SYS_NICE) processes ignore the RLIMIT_RTPRIO limit; as
197 with older kernels, they can make arbitrary changes to scheduling pol‐
198 icy and priority. See getrlimit(2) for further information on
199 RLIMIT_RTPRIO.
200 Response time
202 A blocked high priority process waiting for the I/O has a certain
203 response time before it is scheduled again. The device driver writer
204 can greatly reduce this response time by using a "slow interrupt"
205 interrupt handler.
206 Miscellaneous
208 Child processes inherit the scheduling policy and parameters across a
209 fork(2). The scheduling policy and parameters are preserved across
210 execve(2).
211 Memory locking is usually needed for real-time processes to avoid pag‐
213 ing delays; this can be done with mlock(2) or mlockall(2).
214 Since a non-blocking infinite loop in a process scheduled under
216 SCHED_FIFO or SCHED_RR will block all processes with lower priority
217 forever, a software developer should always keep available on the con‐
218 sole a shell scheduled under a higher static priority than the tested
219 application. This will allow an emergency kill of tested real-time
220 applications that do not block or terminate as expected. See also the
221 description of the RLIMIT_RTTIME resource limit in getrlimit(2).
222 POSIX systems on which sched_setscheduler() and sched_getscheduler()
224 are available define _POSIX_PRIORITY_SCHEDULING in <unistd.h>.
225
227 On success, sched_setscheduler() returns zero. On success,
228 sched_getscheduler() returns the policy for the process (a non-negative
229 integer). On error, -1 is returned, and errno is set appropriately.
230
232 EINVAL The scheduling policy is not one of the recognized policies, or
233 param does not make sense for the policy.
234 EPERM The calling process does not have appropriate privileges.
236 ESRCH The process whose ID is pid could not be found.
238
240 POSIX.1-2001 (but see BUGS below). The SCHED_BATCH and SCHED_IDLE
241 policies are Linux-specific.
242
244 POSIX.1 does not detail the permissions that an unprivileged process
245 requires in order to call sched_setscheduler(), and details vary across
246 systems. For example, the Solaris 7 manual page says that the real or
247 effective user ID of the calling process must match the real user ID or
248 the save set-user-ID of the target process.
249 Originally, Standard Linux was intended as a general-purpose operating
251 system being able to handle background processes, interactive applica‐
252 tions, and less demanding real-time applications (applications that
253 need to usually meet timing deadlines). Although the Linux kernel 2.6
254 allowed for kernel preemption and the newly introduced O(1) scheduler
255 ensures that the time needed to schedule is fixed and deterministic
256 irrespective of the number of active tasks, true real-time computing
257 was not possible up to kernel version 2.6.17.
258 Real-time features in the mainline Linux kernel
260 From kernel version 2.6.18 onwards, however, Linux is gradually becom‐
261 ing equipped with real-time capabilities, most of which are derived
262 from the former realtime-preempt patches developed by Ingo Molnar,
263 Thomas Gleixner, Steven Rostedt, and others. Until the patches have
264 been completely merged into the mainline kernel (this is expected to be
265 around kernel version 2.6.30), they must be installed to achieve the
266 best real-time performance. These patches are named:
267 patch-kernelversion-rtpatchversion
269 and can be downloaded from http://www.kernel.org/pub/linux/ker‐
271 nel/projects/rt/.
272 Without the patches and prior to their full inclusion into the mainline
274 kernel, the kernel configuration offers only the three preemption
275 classes CONFIG_PREEMPT_NONE, CONFIG_PREEMPT_VOLUNTARY, and CONFIG_PRE‐
276 EMPT_DESKTOP which respectively provide no, some, and considerable
277 reduction of the worst-case scheduling latency.
278 With the patches applied or after their full inclusion into the main‐
280 line kernel, the additional configuration item CONFIG_PREEMPT_RT
281 becomes available. If this is selected, Linux is transformed into a
282 regular real-time operating system. The FIFO and RR scheduling poli‐
283 cies that can be selected using sched_setscheduler() are then used to
284 run a process with true real-time priority and a minimum worst-case
285 scheduling latency.
286
288 POSIX says that on success, sched_setscheduler() should return the pre‐
289 vious scheduling policy. Linux sched_setscheduler() does not conform
290 to this requirement, since it always returns 0 on success.
291
293 getpriority(2), mlock(2), mlockall(2), munlock(2), munlockall(2),
294 nice(2), sched_get_priority_max(2), sched_get_priority_min(2),
295 sched_getaffinity(2), sched_getparam(2), sched_rr_get_interval(2),
296 sched_setaffinity(2), sched_setparam(2), sched_yield(2), setprior‐
297 ity(2), capabilities(7), cpuset(7)
298 Programming for the real world - POSIX.4 by Bill O. Gallmeister,
300 O'Reilly & Associates, Inc., ISBN 1-56592-074-0
301 The kernel source file Documentation/scheduler/sched-rt-group.txt
303 (since kernel 2.6.25).
304
306 This page is part of release 3.22 of the Linux man-pages project. A
307 description of the project, and information about reporting bugs, can
308 be found at http://www.kernel.org/doc/man-pages/.
309Linux 2008-11-06 SCHED_SETSCHEDULER(2)