SETSCHEDULER(2) Linux Programmer's Manual SETSCHEDULER(2)
NAME
sched_setscheduler, sched_getscheduler - set and get
scheduling algorithm/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 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
maintains 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 privileges 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
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 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
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 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 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_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 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, 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 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_priority_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 standard
POSIX.1(1990), POSIX.1b(1993), POSIX.1c(1995), and
POSIX.1i(1995).