sched_getscheduler
SETSCHEDULER(2) Linux Programmer’s Manual SETSCHEDULER(2)
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
*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 follow-
ing three scheduling policies are supported under Linux: SCHED_FIFO,
SCHED_RR, and SCHED_OTHER; 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_FIFO and SCHED_RR are intended for spe-
cial 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 find
out the valid priority range for a scheduling policy in a portable way
on all POSIX.1b conforming 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 pri-
ority.
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 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 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 pro-
cesses 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 (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 1003.1
specifies that the process should go to the end of the list.) A pro-
cess 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 pro-
cess 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 pro-
cess and subsequently resumes execution as a running process will com-
plete the unexpired portion of its round robin time quantum. The
length of the time quantum 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-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 the nice or
setpriority system call) and increased for each time quantum the pro-
cess 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.
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 con-
sole 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_PRIORITY_SCHEDULING in <unistd.h>.
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 iden-
tified 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, interactive applications, and soft real-time
applications (applications that need to usually meet timing dead-
lines). 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.
SEE ALSO
sched_setaffinity(2), sched_getaffinity(2), sched_setparam(2),
sched_getparam(2), sched_yield(2), sched_get_priority_max(2),
sched_get_priority_min(2), sched_rr_get_interval(2), nice(2), setpri-
ority(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 standard POSIX.1(1990), POSIX.1b(1993),
POSIX.1c(1995), and POSIX.1i(1995).
Linux 2.4.18 2002-06-25 SETSCHEDULER(2)
