.\" Hey Emacs! This file is -*- nroff -*- source. .\" .\" Copyright (C) Tom Bjorkholm, Markus Kuhn & David A. Wheeler 1996-1999 .\" and Copyright (C) 2007 Carsten Emde .\" and Copyright (C) 2008 Michael Kerrisk .\" .\" This is free documentation; you can redistribute it and/or .\" modify it under the terms of the GNU General Public License as .\" published by the Free Software Foundation; either version 2 of .\" the License, or (at your option) any later version. .\" .\" The GNU General Public License's references to "object code" .\" and "executables" are to be interpreted as the output of any .\" document formatting or typesetting system, including .\" intermediate and printed output. .\" .\" This manual is distributed in the hope that it will be useful, .\" but WITHOUT ANY WARRANTY; without even the implied warranty of .\" MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the .\" GNU General Public License for more details. .\" .\" You should have received a copy of the GNU General Public .\" License along with this manual; if not, write to the Free .\" Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111, .\" USA. .\" .\" 1996-04-01 Tom Bjorkholm .\" First version written .\" 1996-04-10 Markus Kuhn .\" revision .\" 1999-08-18 David A. Wheeler added Note. .\" Modified, 25 Jun 2002, Michael Kerrisk .\" Corrected description of queue placement by sched_setparam() and .\" sched_setscheduler() .\" A couple of grammar clean-ups .\" Modified 2004-05-27 by Michael Kerrisk .\" 2005-03-23, mtk, Added description of SCHED_BATCH. .\" 2007-07-10, Carsten Emde .\" Add text on real-time features that are currently being .\" added to the mainline kernel. .\" 2008-05-07, mtk; Rewrote and restructured various parts of the page to .\" improve readability. .\" 2010-06-19, mtk, documented SCHED_RESET_ON_FORK .\" .\" Worth looking at: http://rt.wiki.kernel.org/index.php .\" .TH SCHED_SETSCHEDULER 2 2012-08-05 "Linux" "Linux Programmer's Manual" .SH NAME sched_setscheduler, sched_getscheduler \- set and get scheduling policy/parameters .SH SYNOPSIS .nf .B #include .sp .BI "int sched_setscheduler(pid_t " pid ", int " policy , .br .BI " const struct sched_param *" param ); .sp .BI "int sched_getscheduler(pid_t " pid ); .sp \fBstruct sched_param { ... int \fIsched_priority\fB; ... }; .fi .SH DESCRIPTION .BR sched_setscheduler () sets both the scheduling policy and the associated parameters for the process whose ID is specified in \fIpid\fP. If \fIpid\fP equals zero, the scheduling policy and parameters of the calling process will be set. The interpretation of the argument \fIparam\fP depends on the selected policy. Currently, Linux supports the following "normal" (i.e., non-real-time) scheduling policies: .TP 14 .BR SCHED_OTHER the standard round-robin time-sharing policy; .\" In the 2.6 kernel sources, SCHED_OTHER is actually called .\" SCHED_NORMAL. .TP .BR SCHED_BATCH for "batch" style execution of processes; and .TP .BR SCHED_IDLE for running .I very low priority background jobs. .PP The following "real-time" policies are also supported, for special time-critical applications that need precise control over the way in which runnable processes are selected for execution: .TP 14 .BR SCHED_FIFO a first-in, first-out policy; and .TP .BR SCHED_RR a round-robin policy. .PP The semantics of each of these policies are detailed below. .BR sched_getscheduler () queries the scheduling policy currently applied to the process identified by \fIpid\fP. If \fIpid\fP equals zero, the policy of the calling process will be retrieved. .\" .SS Scheduling Policies The scheduler is the kernel component that decides which runnable process will be executed by the CPU next. Each process has an associated scheduling policy and a \fIstatic\fP scheduling priority, \fIsched_priority\fP; these are the settings that are modified by .BR sched_setscheduler (). The scheduler makes it decisions based on knowledge of the scheduling policy and static priority of all processes on the system. For processes scheduled under one of the normal scheduling policies (\fBSCHED_OTHER\fP, \fBSCHED_IDLE\fP, \fBSCHED_BATCH\fP), \fIsched_priority\fP is not used in scheduling decisions (it must be specified as 0). Processes scheduled under one of the real-time policies (\fBSCHED_FIFO\fP, \fBSCHED_RR\fP) have a \fIsched_priority\fP value in the range 1 (low) to 99 (high). (As the numbers imply, real-time processes always have higher priority than normal processes.) Note well: POSIX.1-2001 only requires an implementation to support a minimum 32 distinct priority levels for the real-time policies, and some systems supply just this minimum. Portable programs should use .BR sched_get_priority_min (2) and .BR sched_get_priority_max (2) to find the range of priorities supported for a particular policy. Conceptually, the scheduler maintains a list of runnable processes for each possible \fIsched_priority\fP value. In order to determine which process runs next, the scheduler looks for the nonempty list with the highest static priority and selects the process at the head of this list. A process's scheduling policy determines where it will be inserted into the list of processes with equal static priority and how it will move inside this list. All scheduling is preemptive: if a process with a higher static priority becomes ready to run, the currently running process will be preempted and returned to the wait list for its static priority level. The scheduling policy only determines the ordering within the list of runnable processes with equal static priority. .SS SCHED_FIFO: First In-First Out scheduling \fBSCHED_FIFO\fP can only be used with static priorities higher than 0, which means that when a \fBSCHED_FIFO\fP processes becomes runnable, it will always immediately preempt any currently running \fBSCHED_OTHER\fP, \fBSCHED_BATCH\fP, or \fBSCHED_IDLE\fP process. \fBSCHED_FIFO\fP is a simple scheduling algorithm without time slicing. For processes scheduled under the \fBSCHED_FIFO\fP policy, the following rules apply: .IP * 3 A \fBSCHED_FIFO\fP process that has been preempted by another process of higher priority will stay at the head of the list for its priority and will resume execution as soon as all processes of higher priority are blocked again. .IP * When a \fBSCHED_FIFO\fP process becomes runnable, it will be inserted at the end of the list for its priority. .IP * A call to .BR sched_setscheduler () or .BR sched_setparam (2) will put the \fBSCHED_FIFO\fP (or \fBSCHED_RR\fP) process identified by \fIpid\fP at the start of the list if it was runnable. As a consequence, it may preempt the currently running process if it has the same priority. (POSIX.1-2001 specifies that the process should go to the end of the list.) .\" In 2.2.x and 2.4.x, the process is placed at the front of the queue .\" In 2.0.x, the Right Thing happened: the process went to the back -- MTK .IP * A process calling .BR sched_yield (2) will be put at the end of the list. .PP No other events will move a process scheduled under the \fBSCHED_FIFO\fP policy in the wait list of runnable processes with equal static priority. A \fBSCHED_FIFO\fP process runs until either it is blocked by an I/O request, it is preempted by a higher priority process, or it calls .BR sched_yield (2). .SS SCHED_RR: Round Robin scheduling \fBSCHED_RR\fP is a simple enhancement of \fBSCHED_FIFO\fP. Everything described above for \fBSCHED_FIFO\fP also applies to \fBSCHED_RR\fP, except that each process is only allowed to run for a maximum time quantum. If a \fBSCHED_RR\fP process has been running for a time period equal to or longer than the time quantum, it will be put at the end of the list for its priority. A \fBSCHED_RR\fP process that has been preempted by a higher priority process and subsequently resumes execution as a running process will complete the unexpired portion of its round robin time quantum. The length of the time quantum can be retrieved using .BR sched_rr_get_interval (2). .\" On Linux 2.4, the length of the RR interval is influenced .\" by the process nice value -- MTK .\" .SS SCHED_OTHER: Default Linux time-sharing scheduling \fBSCHED_OTHER\fP can only be used at static priority 0. \fBSCHED_OTHER\fP is the standard Linux time-sharing scheduler that is intended for all processes that do not require the special real-time mechanisms. The process to run is chosen from the static priority 0 list based on a \fIdynamic\fP priority that is determined only inside this list. The dynamic priority is based on the nice value (set by .BR nice (2) or .BR setpriority (2)) and increased for each time quantum the process is ready to run, but denied to run by the scheduler. This ensures fair progress among all \fBSCHED_OTHER\fP processes. .\" .SS SCHED_BATCH: Scheduling batch processes (Since Linux 2.6.16.) \fBSCHED_BATCH\fP can only be used at static priority 0. This policy is similar to \fBSCHED_OTHER\fP in that it schedules the process according to its dynamic priority (based on the nice value). The difference is that this policy will cause the scheduler to always assume that the process is CPU-intensive. Consequently, the scheduler will apply a small scheduling penalty with respect to wakeup behaviour, so that this process is mildly disfavored in scheduling decisions. .\" The following paragraph is drawn largely from the text that .\" accompanied Ingo Molnar's patch for the implementation of .\" SCHED_BATCH. This policy is useful for workloads that are noninteractive, but do not want to lower their nice value, and for workloads that want a deterministic scheduling policy without interactivity causing extra preemptions (between the workload's tasks). .\" .SS SCHED_IDLE: Scheduling very low priority jobs (Since Linux 2.6.23.) \fBSCHED_IDLE\fP can only be used at static priority 0; the process nice value has no influence for this policy. This policy is intended for running jobs at extremely low priority (lower even than a +19 nice value with the .B SCHED_OTHER or .B SCHED_BATCH policies). .\" .SS Resetting scheduling policy for child processes Since Linux 2.6.32, the .B SCHED_RESET_ON_FORK flag can be ORed in .I policy when calling .BR sched_setscheduler (). As a result of including this flag, children created by .BR fork (2) do not inherit privileged scheduling policies. This feature is intended for media-playback applications, and can be used to prevent applications evading the .BR RLIMIT_RTTIME resource limit (see .BR getrlimit (2)) by creating multiple child processes. More precisely, if the .BR SCHED_RESET_ON_FORK flag is specified, the following rules apply for subsequently created children: .IP * 3 If the calling process has a scheduling policy of .B SCHED_FIFO or .BR SCHED_RR , the policy is reset to .BR SCHED_OTHER in child processes. .IP * If the calling process has a negative nice value, the nice value is reset to zero in child processes. .PP After the .BR SCHED_RESET_ON_FORK flag has been enabled, it can only be reset if the process has the .BR CAP_SYS_NICE capability. This flag is disabled in child processes created by .BR fork (2). The .B SCHED_RESET_ON_FORK flag is visible in the policy value returned by .BR sched_getscheduler () .\" .SS Privileges and resource limits In Linux kernels before 2.6.12, only privileged .RB ( CAP_SYS_NICE ) processes can set a nonzero static priority (i.e., set a real-time scheduling policy). The only change that an unprivileged process can make is to set the .B SCHED_OTHER policy, and this can only be done if the effective user ID of the caller of .BR sched_setscheduler () matches the real or effective user ID of the target process (i.e., the process specified by .IR pid ) whose policy is being changed. Since Linux 2.6.12, the .B RLIMIT_RTPRIO resource limit defines a ceiling on an unprivileged process's static priority for the .B SCHED_RR and .B SCHED_FIFO policies. The rules for changing scheduling policy and priority are as follows: .IP * 3 If an unprivileged process has a nonzero .B RLIMIT_RTPRIO soft limit, then it can change its scheduling policy and priority, subject to the restriction that the priority cannot be set to a value higher than the maximum of its current priority and its .B RLIMIT_RTPRIO soft limit. .IP * If the .B RLIMIT_RTPRIO soft limit is 0, then the only permitted changes are to lower the priority, or to switch to a non-real-time policy. .IP * Subject to the same rules, another unprivileged process can also make these changes, as long as the effective user ID of the process making the change matches the real or effective user ID of the target process. .IP * Special rules apply for the .BR SCHED_IDLE . In Linux kernels before 2.6.39, an unprivileged process operating under this policy cannot change its policy, regardless of the value of its .BR RLIMIT_RTPRIO resource limit. In Linux kernels since 2.6.39, .\" commit c02aa73b1d18e43cfd79c2f193b225e84ca497c8 an unprivileged process can switch to either the .BR SCHED_BATCH or the .BR SCHED_NORMAL policy so long as its nice value falls within the range permitted by its .BR RLIMIT_NICE resource limit (see .BR getrlimit (2)). .PP Privileged .RB ( CAP_SYS_NICE ) processes ignore the .B RLIMIT_RTPRIO limit; as with older kernels, they can make arbitrary changes to scheduling policy and priority. See .BR getrlimit (2) for further information on .BR RLIMIT_RTPRIO . .SS Response time A blocked high priority process waiting for the I/O has a certain response time before it is scheduled again. The device driver writer can greatly reduce this response time by using a "slow interrupt" interrupt handler. .\" as described in .\" .BR request_irq (9). .SS Miscellaneous Child processes inherit the scheduling policy and parameters across a .BR fork (2). The scheduling policy and parameters are preserved across .BR execve (2). Memory locking is usually needed for real-time processes to avoid paging delays; this can be done with .BR mlock (2) or .BR mlockall (2). Since a nonblocking infinite loop in a process scheduled under \fBSCHED_FIFO\fP or \fBSCHED_RR\fP will block all processes with lower priority forever, a software developer should always keep available on the console a shell scheduled under a higher static priority than the tested application. This will allow an emergency kill of tested real-time applications that do not block or terminate as expected. See also the description of the .BR RLIMIT_RTTIME resource limit in .BR getrlimit (2). POSIX systems on which .BR sched_setscheduler () and .BR sched_getscheduler () are available define .B _POSIX_PRIORITY_SCHEDULING in \fI\fP. .SH "RETURN VALUE" On success, .BR sched_setscheduler () returns zero. On success, .BR sched_getscheduler () returns the policy for the process (a nonnegative integer). On error, \-1 is returned, and .I errno is set appropriately. .SH ERRORS .TP .B EINVAL The scheduling \fIpolicy\fP is not one of the recognized policies, \fIparam\fP is NULL, or \fIparam\fP does not make sense for the \fIpolicy\fP. .TP .B EPERM The calling process does not have appropriate privileges. .TP .B ESRCH The process whose ID is \fIpid\fP could not be found. .SH "CONFORMING TO" POSIX.1-2001 (but see BUGS below). The \fBSCHED_BATCH\fP and \fBSCHED_IDLE\fP policies are Linux-specific. .SH NOTES POSIX.1 does not detail the permissions that an unprivileged process requires in order to call .BR sched_setscheduler (), and details vary across systems. For example, the Solaris 7 manual page says that the real or effective user ID of the calling process must match the real user ID or the save set-user-ID of the target process. .PP Originally, Standard Linux was intended as a general-purpose operating system being able to handle background processes, interactive applications, and less demanding real-time applications (applications that need to usually meet timing deadlines). Although the Linux kernel 2.6 allowed for kernel preemption and the newly introduced O(1) scheduler ensures that the time needed to schedule is fixed and deterministic irrespective of the number of active tasks, true real-time computing was not possible up to kernel version 2.6.17. .SS Real-time features in the mainline Linux kernel .\" FIXME . Probably this text will need some minor tweaking .\" by about the time of 2.6.30; ask Carsten Emde about this then. From kernel version 2.6.18 onward, however, Linux is gradually becoming equipped with real-time capabilities, most of which are derived from the former .I realtime-preempt patches developed by Ingo Molnar, Thomas Gleixner, Steven Rostedt, and others. Until the patches have been completely merged into the mainline kernel (this is expected to be around kernel version 2.6.30), they must be installed to achieve the best real-time performance. These patches are named: .in +4n .nf patch-\fIkernelversion\fP-rt\fIpatchversion\fP .fi .in .PP and can be downloaded from .UR http://www.kernel.org\:/pub\:/linux\:/kernel\:/projects\:/rt/ .UE . Without the patches and prior to their full inclusion into the mainline kernel, the kernel configuration offers only the three preemption classes .BR CONFIG_PREEMPT_NONE , .BR CONFIG_PREEMPT_VOLUNTARY , and .B CONFIG_PREEMPT_DESKTOP which respectively provide no, some, and considerable reduction of the worst-case scheduling latency. With the patches applied or after their full inclusion into the mainline kernel, the additional configuration item .B CONFIG_PREEMPT_RT becomes available. If this is selected, Linux is transformed into a regular real-time operating system. The FIFO and RR scheduling policies that can be selected using .BR sched_setscheduler () are then used to run a process with true real-time priority and a minimum worst-case scheduling latency. .SH BUGS POSIX says that on success, .BR sched_setscheduler () should return the previous scheduling policy. Linux .BR sched_setscheduler () does not conform to this requirement, since it always returns 0 on success. .SH "SEE ALSO" .ad l .nh .BR getpriority (2), .BR mlock (2), .BR mlockall (2), .BR munlock (2), .BR munlockall (2), .BR nice (2), .BR sched_get_priority_max (2), .BR sched_get_priority_min (2), .BR sched_getaffinity (2), .BR sched_getparam (2), .BR sched_rr_get_interval (2), .BR sched_setaffinity (2), .BR sched_setparam (2), .BR sched_yield (2), .BR setpriority (2), .BR capabilities (7), .BR cpuset (7) .ad j .PP .I Programming for the real world \- POSIX.4 by Bill O. Gallmeister, O'Reilly & Associates, Inc., ISBN 1-56592-074-0. .PP .I Documentation/scheduler/sched-rt-group.txt in the Linux kernel source tree (since kernel 2.6.25). .SH COLOPHON This page is part of release 3.44 of the Linux .I man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.