SCHED_SETSCHEDULER
Section: Linux Programmer's Manual (2)
Updated: 2010-06-19
NAME
sched_setscheduler, sched_getscheduler -
set and get scheduling policy/parameters
SYNOPSIS
#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;
...
};
DESCRIPTION
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.
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 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: First In-First Out scheduling
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: Round Robin scheduling
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: Default Linux time-sharing scheduling
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.
SCHED_BATCH: Scheduling batch 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).
SCHED_IDLE: Scheduling very low priority jobs
(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).
Resetting scheduling policy for child processes
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()
Privileges and resource limits
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:
an unprivileged process operating under this policy cannot
change its policy, regardless of the value of its
RLIMIT_RTPRIO
resource limit.
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.
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.
Miscellaneous
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>.
RETURN VALUE
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.
ERRORS
- EINVAL
-
The scheduling policy is not one of the recognized policies,
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.
CONFORMING TO
POSIX.1-2001 (but see BUGS below).
The SCHED_BATCH and SCHED_IDLE policies are Linux-specific.
NOTES
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.
Real-time features in the mainline Linux kernel
From kernel version 2.6.18 onwards, 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.
BUGS
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.
SEE ALSO
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).
COLOPHON
This page is part of release 3.27 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/.
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- Scheduling Policies
-
- SCHED_FIFO: First In-First Out scheduling
-
- SCHED_RR: Round Robin scheduling
-
- SCHED_OTHER: Default Linux time-sharing scheduling
-
- SCHED_BATCH: Scheduling batch processes
-
- SCHED_IDLE: Scheduling very low priority jobs
-
- Resetting scheduling policy for child processes
-
- Privileges and resource limits
-
- Response time
-
- Miscellaneous
-
- RETURN VALUE
-
- ERRORS
-
- CONFORMING TO
-
- NOTES
-
- Real-time features in the mainline Linux kernel
-
- BUGS
-
- SEE ALSO
-
- COLOPHON
-
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Time: 07:34:57 GMT, March 26, 2013