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SETSCHEDULER(2) Linux Programmer’s Manual SETSCHEDULER(2)
NAME
sched_setscheduler, sched_getscheduler - set and get scheduling algorithm/parame-
ters
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 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 exe-
cuted by the CPU next. The Linux scheduler offers three different scheduling poli-
cies, one for normal processes and two for real-time applications. A static prior-
ity 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 applica-
tions 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 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 pro-
cesses 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 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 pol-
icy, 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_set-
param 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 1003.1 specifies that the pro-
cess should go to the end of the list.) 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 prior-
ity. 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 pri-
ority 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-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 set-
priority 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.
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 prior-
ity 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 pro-
cesses 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 parame-
ter 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 pro-
cesses, 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 guaran-
teed 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, con-
sider 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), 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 & Asso-
ciates, 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)
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