| NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | NOTES | BUGS | SEE ALSO | COLOPHON | The Linux Programming Interface |
SCHED_SETSCHEDULER(2) Linux Programmer's Manual SCHED_SETSCHEDULER(2)
sched_setscheduler, sched_getscheduler - set and get scheduling pol-
icy/parameters
#include <sched.h>
int sched_setscheduler(pid_t pid, int policy,
const struct sched_param *param);
int sched_getscheduler(pid_t pid);
struct sched_param {
...
int sched_priority;
...
};
sched_setscheduler() sets both the scheduling policy and the associated
parameters for the process whose ID is specified in pid. If pid equals
zero, the scheduling policy and parameters of the calling process will be
set. The interpretation of the argument param depends on the selected
policy. Currently, Linux supports the following "normal" (i.e., non-real-
time) scheduling policies:
SCHED_OTHER the standard round-robin time-sharing policy;
SCHED_BATCH for "batch" style execution of processes; and
SCHED_IDLE for running very low priority background jobs.
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:
SCHED_FIFO a first-in, first-out policy; and
SCHED_RR a round-robin policy.
The semantics of each of these policies are detailed below.
sched_getscheduler() queries the scheduling policy currently applied to the
process identified by pid. If pid equals zero, the policy of the calling
process will be retrieved.
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 static scheduling priority, sched_priority; these
are the settings that are modified by 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
(SCHED_OTHER, SCHED_IDLE, SCHED_BATCH), sched_priority is not used in
scheduling decisions (it must be specified as 0).
Processes scheduled under one of the real-time policies (SCHED_FIFO,
SCHED_RR) have a sched_priority 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 sched_get_priority_min(2) and 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 sched_priority 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.
SCHED_FIFO can only be used with static priorities higher than 0, which
means that when a SCHED_FIFO processes becomes runnable, it will always
immediately preempt any currently running SCHED_OTHER, SCHED_BATCH, or
SCHED_IDLE process. SCHED_FIFO is a simple scheduling algorithm without
time slicing. For processes scheduled under the SCHED_FIFO policy, the
following rules apply:
* A SCHED_FIFO 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.
* When a SCHED_FIFO process becomes runnable, it will be inserted at the
end of the list for its priority.
* A call to sched_setscheduler() or sched_setparam(2) will put the
SCHED_FIFO (or SCHED_RR) process identified by pid 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.)
* A process calling sched_yield(2) will be put at the end of the list.
No other events will move a process scheduled under the SCHED_FIFO policy
in the wait list of runnable processes with equal static priority.
A SCHED_FIFO process runs until either it is blocked by an I/O request, it
is preempted by a higher priority process, or it calls sched_yield(2).
SCHED_RR is a simple enhancement of SCHED_FIFO. Everything described above
for SCHED_FIFO also applies to SCHED_RR, except that each process is only
allowed to run for a maximum time quantum. If a SCHED_RR 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 SCHED_RR 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 sched_rr_get_interval(2).
SCHED_OTHER can only be used at static priority 0. SCHED_OTHER 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 dynamic priority that
is determined only inside this list. The dynamic priority is based on the
nice value (set by nice(2) or 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 SCHED_OTHER processes.
(Since Linux 2.6.16.) SCHED_BATCH can only be used at static priority 0.
This policy is similar to SCHED_OTHER 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.
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).
(Since Linux 2.6.23.) SCHED_IDLE 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 SCHED_OTHER or SCHED_BATCH policies).
Since Linux 2.6.32, the SCHED_RESET_ON_FORK flag can be ORed in policy when
calling sched_setscheduler(). As a result of including this flag, children
created by 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 RLIMIT_RTTIME resource limit (see
getrlimit(2)) by creating multiple child processes.
More precisely, if the SCHED_RESET_ON_FORK flag is specified, the following
rules apply for subsequently created children:
* If the calling process has a scheduling policy of SCHED_FIFO or
SCHED_RR, the policy is reset to SCHED_OTHER in child processes.
* If the calling process has a negative nice value, the nice value is
reset to zero in child processes.
After the SCHED_RESET_ON_FORK flag has been enabled, it can only be reset
if the process has the CAP_SYS_NICE capability. This flag is disabled in
child processes created by fork(2).
The SCHED_RESET_ON_FORK flag is visible in the policy value returned by
sched_getscheduler()
In Linux kernels before 2.6.12, only privileged (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 SCHED_OTHER policy, and this can only be done if the effective user ID
of the caller of sched_setscheduler() matches the real or effective user ID
of the target process (i.e., the process specified by pid) whose policy is
being changed.
Since Linux 2.6.12, the RLIMIT_RTPRIO resource limit defines a ceiling on
an unprivileged process's static priority for the SCHED_RR and SCHED_FIFO
policies. The rules for changing scheduling policy and priority are as
follows:
* If an unprivileged process has a nonzero 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 RLIMIT_RTPRIO soft limit.
* If the 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.
* 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.
* Special rules apply for the 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 RLIMIT_RTPRIO resource limit. In
Linux kernels since 2.6.39, an unprivileged process can switch to either
the SCHED_BATCH or the SCHED_NORMAL policy so long as its nice value
falls within the range permitted by its RLIMIT_NICE resource limit (see
getrlimit(2)).
Privileged (CAP_SYS_NICE) processes ignore the RLIMIT_RTPRIO limit; as with
older kernels, they can make arbitrary changes to scheduling policy and
priority. See getrlimit(2) for further information on RLIMIT_RTPRIO.
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.
Child processes inherit the scheduling policy and parameters across a
fork(2). The scheduling policy and parameters are preserved across
execve(2).
Memory locking is usually needed for real-time processes to avoid paging
delays; this can be done with mlock(2) or mlockall(2).
Since a nonblocking infinite loop in a process scheduled under SCHED_FIFO
or SCHED_RR 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
RLIMIT_RTTIME resource limit in getrlimit(2).
POSIX systems on which sched_setscheduler() and sched_getscheduler() are
available define _POSIX_PRIORITY_SCHEDULING in <unistd.h>.
On success, sched_setscheduler() returns zero. On success,
sched_getscheduler() returns the policy for the process (a nonnegative
integer). On error, -1 is returned, and errno is set appropriately.
EINVAL The scheduling policy is not one of the recognized policies, param
is NULL, or param does not make sense for the policy.
EPERM The calling process does not have appropriate privileges.
ESRCH The process whose ID is pid could not be found.
POSIX.1-2001 (but see BUGS below). The SCHED_BATCH and SCHED_IDLE policies
are Linux-specific.
POSIX.1 does not detail the permissions that an unprivileged process
requires in order to call 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.
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.
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 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:
patch-kernelversion-rtpatchversion
and can be downloaded from
http://www.kernel.org/pub/linux/kernel/projects/rt/.
Without the patches and prior to their full inclusion into the mainline
kernel, the kernel configuration offers only the three preemption classes
CONFIG_PREEMPT_NONE, CONFIG_PREEMPT_VOLUNTARY, and 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 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 sched_setscheduler() are then used to run a process with
true real-time priority and a minimum worst-case scheduling latency.
POSIX says that on success, sched_setscheduler() should return the previous
scheduling policy. Linux sched_setscheduler() does not conform to this
requirement, since it always returns 0 on success.
getpriority(2), mlock(2), mlockall(2), munlock(2), munlockall(2), nice(2),
sched_get_priority_max(2), sched_get_priority_min(2), sched_getaffinity(2),
sched_getparam(2), sched_rr_get_interval(2), sched_setaffinity(2),
sched_setparam(2), sched_yield(2), setpriority(2), capabilities(7),
cpuset(7)
Programming for the real world - POSIX.4 by Bill O. Gallmeister, O'Reilly &
Associates, Inc., ISBN 1-56592-074-0
The kernel source file Documentation/scheduler/sched-rt-group.txt (since
kernel 2.6.25).
This page is part of release 3.41 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can be
found at http://www.kernel.org/doc/man-pages/.
Linux 2011-09-19 SCHED_SETSCHEDULER(2)
HTML rendering created 2012-05-11 by Michael Kerrisk,
author of
The Linux Programming Interface,
maintainer of the
Linux man-pages project