SETSCHEDULER(2) Linux Programmer's Manual SETSCHEDULER(2)
NAME
sched_setscheduler, sched_getscheduler - set and get scheduling algorithm/parameters
SYNOPSIS
#include
int sched_setscheduler(pid_t pid, int policy, const struct sched_param *p);
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 p depends on the selected policy. Currently, the following three
scheduling policies are supported under Linux: SCHED_FIFO, SCHED_RR, and SCHED_OTHER; their
respective semantics is 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 main
tains a list of runnable processes 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_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 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. Only processes with superuser privi
leges can get a static priority higher than 0 and can therefore be scheduled under SCHED_FIFO or
SCHED_RR. The system calls sched_get_priority_min and sched_get_priority_max can be used to to
find out the valid priority range for a scheduling policy in a portable way on all POSIX.1b con
forming systems.
All scheduling is preemptive: If a process with a higher static priority 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 priority.
SCHED_FIFO: First In-First out scheduling
SCHED_FIFO can only be used with static priorities higher than 0, that means that when a
SCHED_FIFO processes becomes runnable, it will always preempt immediately any currently running
normal SCHED_OTHER process. SCHED_FIFO is a simple scheduling algorithm without time slicing. For
processes scheduled under the SCHED_FIFO policy, the following 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 will put the SCHED_FIFO
process identified by pid at the end of the list if it was runnable. A process calling
sched_yield 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.
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 quan
tum, 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 quan
tum can be retrieved by sched_rr_get_interval.
SCHED_OTHER: Default Linux time-sharing scheduling
SCHED_OTHER can only be used at static priority 0. SCHED_OTHER is the standard Linux time-shar
ing scheduler that is intended for all processes that do not require special static priority
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 level (set by the nice or setpriority system call) 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.
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 request_irq(9).
Miscellaneous
Child processes inherit the scheduling algorithm and parameters across a fork.
Memory locking is usually needed for real-time processes to avoid paging delays, this can be done
with mlock or mlockall.
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. As SCHED_FIFO and SCHED_RR processes can preempt other processes forever, only root
processes are allowed to activate these policies under Linux.
POSIX systems on which sched_setscheduler and sched_getscheduler are available define _POSIX_PRI
ORITY_SCHEDULING in .
RETURN VALUE
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, errno is set appropriately.
ERRORS
ESRCH The process whose ID is pid could not be found.
EPERM The calling process does not have appropriate privileges. Only root processes are allowed
to activate the SCHED_FIFO and SCHED_RR policies. The process calling sched_setscheduler
needs an effective uid equal to the euid or uid of the process identified by pid, or it
must be a superuser process.
EINVAL The scheduling policy is not one of the recognized policies, or the parameter p does not
make sense for the policy.
CONFORMING TO
POSIX.1b (formerly POSIX.4)
BUGS
As of linux-1.3.81, SCHED_RR has not yet been tested carefully and might not behave exactly as
described or required by POSIX.1b.
NOTE
Standard Linux is a general-purpose operating system and can handle background processes, inter
active applications, and soft real-time applications (applications that need to usually meet tim
ing deadlines). This man page is directed at these kinds of applications.
BUGS
As of linux-1.3.81, SCHED_RR has not yet been tested carefully and might not behave exactly as
described or required by POSIX.1b.
NOTE
Standard Linux is a general-purpose operating system and can handle background processes, inter
active applications, and soft real-time applications (applications that need to usually meet tim
ing 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 maximize
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) produce 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 use a different operating system designed
specifically for hard real-time applications.
SEE ALSO
sched_setparam(2), sched_getparam(2), sched_yield(2), sched_get_priority_max(2), sched_get_prior
ity_min(2), nice(2), setpriority(2), getpriority(2), mlockall(2), munlockall(2), mlock(2),
munlock(2).
Programming for the real world - POSIX.4 by Bill O. Gallmeister, O'Reilly & Associates, Inc.,
ISBN 1-56592-074-0
IEEE Std 1003.1b-1993 (POSIX.1b standard)
ISO/IEC 9945-1:1996 - This is the new 1996 revision of POSIX.1 which contains in one single stan
dard POSIX.1(1990), POSIX.1b(1993), POSIX.1c(1995), and POSIX.1i(1995).
Linux 2.2.11 1999-08-18 1