man-pages/man2/sched_setscheduler.2

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.\" Copyright (C) Tom Bjorkholm, Markus Kuhn & David A. Wheeler 1996-1999
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.\" 1996-04-01 Tom Bjorkholm <tomb@mydata.se>
.\"            First version written
.\" 1996-04-10 Markus Kuhn <mskuhn@cip.informatik.uni-erlangen.de>
.\"            revision
.\" 1999-08-18 David A. Wheeler <dwheeler@ida.org> added Note.
.\" Modified, 25 Jun 2002, Michael Kerrisk <mtk-manpages@gmx.net>
.\"	Corrected description of queue placement by sched_setparam() and 
.\"		sched_setscheduler()
.\"	A couple of grammar clean-ups
.\" Modified 2004-05-27 by Michael Kerrisk <mtk-manpages@gmx.net>
.\"
.TH SETSCHEDULER 2 2005-07-28 "Linux 2.6.12" "Linux Programmer's Manual"
.SH NAME
sched_setscheduler, sched_getscheduler \-
set and get scheduling algorithm/parameters
.SH SYNOPSIS
.B #include <sched.h>
.sp
.BI "int sched_setscheduler(pid_t " pid ", int " policy ,
.BI "const struct sched_param *" p );
.sp
.BI "int sched_getscheduler(pid_t " pid );
.sp
.nf
.ta 4n
\fBstruct sched_param {
	...
	int \fIsched_priority\fB;
	...
};
.ta
.fi
.SH DESCRIPTION
.B sched_setscheduler
sets both the scheduling policy and the associated parameters for the
process identified by \fIpid\fP. If \fIpid\fP equals zero, the
scheduler of the calling process will be set. The interpretation of
the parameter \fIp\fP depends on the selected policy. Currently, the
following three scheduling policies are supported under Linux:
.IR SCHED_FIFO , 
.IR SCHED_RR ,
and 
.IR SCHED_OTHER ;
their respective semantics are described below.

.B sched_getscheduler
queries the scheduling policy currently applied to the process
identified by \fIpid\fP. If \fIpid\fP equals zero, the policy of the
calling process will be retrieved.


.SS 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 \fIsched_priority\fP
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 \fIsched_priority\fP value, and
\fIsched_priority\fP 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.

\fISCHED_OTHER\fP is the default universal time-sharing scheduler
policy used by most processes, \fISCHED_FIFO\fP and \fISCHED_RR\fP 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 \fISCHED_OTHER\fP must be assigned
the static priority 0, processes scheduled under \fISCHED_FIFO\fP or
\fISCHED_RR\fP can have a static priority in the range 1 to 99. 
The system calls \fBsched_get_priority_min\fP and
\fBsched_get_priority_max\fP 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
priority.

.SS SCHED_FIFO: First In-First Out scheduling
\fISCHED_FIFO\fP can only be used with static priorities higher than
0, which means that when a \fISCHED_FIFO\fP processes becomes runnable,
it will always preempt immediately any currently running normal
\fISCHED_OTHER\fP process. \fISCHED_FIFO\fP is a simple scheduling
algorithm without time slicing. For processes scheduled under the
\fISCHED_FIFO\fP policy, the following rules are applied: A
\fISCHED_FIFO\fP 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 \fISCHED_FIFO\fP process becomes runnable, it
will be inserted at the end of the list for its priority. A call to
\fBsched_setscheduler\fP or \fBsched_setparam\fP will put the
\fISCHED_FIFO\fP (or \fISCHED_RR\fP) process identified by
\fIpid\fP 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 process should go to the end
of the list.)
.\" In 2.2.x and 2.4.x, the process is placed at the front of the queue
.\" In 2.0.x, the Right Thing happened: the process went to the back -- MTK
A process calling \fBsched_yield\fP will be
put at the end of the list. No other events will move a process
scheduled under the \fISCHED_FIFO\fP policy in the wait list of
runnable processes with equal static priority. A \fISCHED_FIFO\fP
process runs until either it is blocked by an I/O request, it is
preempted by a higher priority process, or it calls \fBsched_yield\fP.

.SS SCHED_RR: Round Robin scheduling
\fISCHED_RR\fP is a simple enhancement of \fISCHED_FIFO\fP. Everything
described above for \fISCHED_FIFO\fP also applies to \fISCHED_RR\fP,
except that each process is only allowed to run for a maximum time
quantum. If a \fISCHED_RR\fP 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 \fISCHED_RR\fP 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 \fBsched_rr_get_interval\fP.
.\" On Linux 2.4, the length of the RR interval is influenced
.\" by the process nice value -- MTK

.SS SCHED_OTHER: Default Linux time-sharing scheduling
\fISCHED_OTHER\fP can only be used at static priority 0.
\fISCHED_OTHER\fP 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 \fBnice\fP or \fBsetpriority\fP 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 \fISCHED_OTHER\fP
processes.

.SS Privileges and resource limits
.\" FIXME -- make some general statement about Unix implementations
.\" A process calling 
.\" .BR sched_setscheduler
.\" needs an effective user ID equal to the real user ID or effective 
.\" user ID of the process identified by
.\" .IR pid ,
.\" or it must be privileged (Linux: have the
.\" .B CAP_SYS_NICE
.\" capability).
.\" 
In Linux kernels before 2.6.12, only privileged
.RB ( CAP_SYS_NICE )
processes can set a non-zero static priority.
The only change that an unprivileged process can make is to set the
.B SCHED_OTHER
policy, and this can only be done if the effective user ID of the caller of
.BR sched_setscheduler
matches the real or effective user ID of the target process 
(i.e., the process specified by
.IR pid )
whose policy is being changed.

Since Linux 2.6.12, the
.B RLIMIT_RTPRIO
resource limit defines a ceiling on an unprivileged process's
priority for the
.B SCHED_RR
and
.BR SCHED_FIFO 
policies.
If an unprivileged process has a non-zero 
.B 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 
.B RLIMIT_RTPRIO
soft limit.
If the 
.B 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
.BR getrlimit (2)
for further information on
.BR RLIMIT_RTPRIO .
Privileged
.RB ( CAP_SYS_NICE )
processes ignore this limit; as with older older kernels,
they can make arbitrary changes to scheduling policy and priority.

.SS 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
.\" .BR request_irq (9).

.SS Miscellaneous
Child processes inherit the scheduling algorithm and parameters across a
.BR fork .

Memory locking is usually needed for real-time processes to avoid 
paging delays, this can be done with
.B mlock 
or 
.BR mlockall .

As a non-blocking end-less loop in a process scheduled under
\fISCHED_FIFO\fP or \fISCHED_RR\fP 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
.B sched_setscheduler
and
.B sched_getscheduler
are available define
.I _POSIX_PRIORITY_SCHEDULING
in <unistd.h>.

.SH "RETURN VALUE"
On success,
.BR sched_setscheduler
returns zero.  
On success,
.BR sched_getscheduler
returns the policy for the process (a non-negative integer).  
On error, \-1 is returned,
.I errno
is set appropriately.
.SH ERRORS
.TP
.B EINVAL
The scheduling \fIpolicy\fP is not one of the recognized policies,
or the parameter \fIp\fP does not make sense for the \fIpolicy\fP.
.TP
.B EPERM
The calling process does not have appropriate privileges.
.TP
.B ESRCH
The process whose ID is \fIpid\fP could not be found.
.SH "CONFORMING TO"
POSIX.1b (formerly POSIX.4)
.SH 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.
.PP
Standard Linux is
.I 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 RTAI (http://www.rtai.org)
or use a different operating system
designed specifically for hard real-time applications.
.SH "SEE ALSO"
.BR getpriority (2),
.BR mlock (2),
.BR mlockall (2),
.BR munlock (2),
.BR munlockall (2),
.BR nice (2),
.BR sched_get_priority_max (2),
.BR sched_get_priority_min (2),
.BR sched_getaffinity (2),
.BR sched_getparam (2),
.BR sched_rr_get_interval (2),
.BR sched_setaffinity (2),
.BR sched_setparam (2),
.BR sched_yield (2),
.BR setpriority (2),
.BR capabilities (7)
.PP
.I Programming for the real world \- POSIX.4
by Bill O. Gallmeister, O'Reilly & Associates, Inc., ISBN 1-56592-074-0
.br
.I IEEE Std 1003.1b-1993
(POSIX.1b standard)
.br
.I 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).