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NAME
sched_setscheduler, sched_getscheduler - set and get scheduling algo‐
rithm/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 identified by pid. If pid equals zero, the
scheduler of the calling process will be set. The interpretation of
the parameter param depends on the selected policy. Currently, the
following three scheduling policies are supported under Linux:
SCHED_FIFO, SCHED_RR, SCHED_OTHER, and SCHED_BATCH; their respective
semantics are described 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 part that decides which runnable process
will be executed by the CPU next. The Linux scheduler offers three
different scheduling policies, one for normal processes and two for
real-time applications. A static priority value sched_priority is
assigned to each process and this value can be changed only via system
calls. Conceptually, the scheduler maintains a list of runnable pro‐
cesses for each possible sched_priority value, and sched_priority can
have a value in the range 0 to 99. In order to determine the process
that runs next, the Linux scheduler looks for the non-empty list with
the highest static priority and takes the process at the head of this
list. The scheduling policy determines for each process, where it will
be inserted into the list of processes with equal static priority and
how it will move inside this list.
SCHED_OTHER is the default universal time-sharing scheduler policy used
by most processes. SCHED_BATCH is intended for "batch" style execution
of processes. SCHED_FIFO and SCHED_RR are intended for special time-
critical applications that need precise control over the way in which
runnable processes are selected for execution.
Processes scheduled with SCHED_OTHER or SCHED_BATCH must be assigned
the static priority 0. Processes scheduled under SCHED_FIFO or
SCHED_RR can have a static priority in the range 1 to 99. The system
calls sched_get_priority_min(2) and sched_get_priority_max(2) can be
used to find out the valid priority range for a scheduling policy in a
portable way on all POSIX.1-2001 conforming systems.
All scheduling is preemptive: If a process with a higher static prior‐
ity gets ready to run, the current process will be preempted and
returned into its wait list. The scheduling policy only determines the
ordering within the list of runnable processes with equal static prior‐
ity.
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 or SCHED_BATCH
process. SCHED_FIFO is a simple scheduling algorithm without time
slicing. For processes scheduled under the SCHED_FIFO policy, the fol‐
lowing rules are applied: 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 cur‐
rently running process if it has the same priority. (POSIX.1-2001
specifies that the process should go to the end of the list.) A pro‐
cess 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 pro‐
cess 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 pro‐
cesses that do not require special static priority real-time mecha‐
nisms. 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 level (set by nice(2) or set
priority(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, except 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 so
that this process is mildly disfavored in scheduling decisions. This
policy is useful for workloads that are non-interactive, but do not
want to lower their nice value, and for workloads that want a determin‐
istic scheduling policy without interactivity causing extra preemptions
(between the workload’s tasks).
Privileges and resource limits
In Linux kernels before 2.6.12, only privileged (CAP_SYS_NICE) pro‐
cesses can set a non-zero static priority. 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 tar‐
get 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 priority for the SCHED_RR and SCHED_FIFO
policies. If an unprivileged process has a non-zero 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 RLIMIT_RTPRIO soft limit. If the RLIMIT_RTPRIO soft limit is
0, then the only permitted change is to lower the priority. 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.
See getrlimit(2) for further information on RLIMIT_RTPRIO. Privileged
(CAP_SYS_NICE) processes ignore this limit; as with older kernels, they
can make arbitrary changes to scheduling policy and priority.
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 algorithm and parameters across
a fork(2). The scheduling algorithm and parameters are preserved
across execve(2).
Memory locking is usually needed for real-time processes to avoid pag‐
ing delays, this can be done with mlock(2) or mlockall(2).
As a non-blocking end-less 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.
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 non-negative
integer). On error, -1 is returned, and errno is set appropriately.
ERRORS
EINVAL The scheduling policy is not one of the recognized policies, or
the parameter 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. The SCHED_BATCH policy is Linux specific.
NOTES
POSIX.1 does not detail the permissions that an unprivileged process
requires in order to call sched_setschedule(), and details vary across
systems. For example, the Solaris 7 manual page says that the real of
effective user ID of the calling process match the real user ID or the
save set-user-ID of the target process.
Standard Linux is a general-purpose operating system and can handle
background processes, interactive applications, and soft real-time
applications (applications that need to usually meet timing deadlines).
This man page is directed at these kinds of applications.
Standard Linux is not designed to support hard real-time applications,
that is, applications in which deadlines (often much shorter than a
second) must be guaranteed or the system will fail catastrophically.
Like all general-purpose operating systems, Linux is designed to maxi‐
mize average case performance instead of worst case performance.
Linux’s worst case performance for interrupt handling is much poorer
than its average case, its various kernel locks (such as for SMP) pro‐
duce long maximum wait times, and many of its performance improvement
techniques decrease average time by increasing worst-case time. For
most situations, that’s what you want, but if you truly are developing
a hard real-time application, consider using hard real-time extensions
to Linux such as RTLinux (http://www.rtlinux.org) or RTAI
(http://www.rtai.org) or use a different operating system designed
specifically for hard real-time applications.
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), setprior
ity(2), capabilities(7)
Programming for the real world - POSIX.4 by Bill O. Gallmeister,
O’Reilly & Associates, Inc., ISBN 1-56592-074-0