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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 *param);
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 param depends on the
selected policy. Currently, the following three scheduling policies are supported
under Linux: SCHED_FIFO, SCHED_RR, SCHED_OTHER, and SCHED_BATCH; 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_BATCH is intended for "batch" style execution of 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 exe-
cution.
Processes scheduled with SCHED_OTHER or SCHED_BATCH 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. 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.1-2001 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 immediately preempt
any currently running SCHED_OTHER or SCHED_BATCH 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 (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.1-2001 specifies
that the process 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 pro-
cess 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 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 using sched_rr_get_interval(2).
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 nice(2) or set-
priority(2)) 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.
SCHED_BATCH: Scheduling batch processes
(Since Linux 2.6.16.) SCHED_BATCH can only be used at static priority 0. This
policy is similar to SCHED_OTHER, except that this policy will cause the scheduler
to always assume that the process is CPU-intensive. Consequently, the scheduler
will apply a small scheduling penalty so that this process is mildly disfavoured in
scheduling decisions. This policy is useful for workloads that are non-interac-
tive, but do not want to lower their nice value, and for workloads that want a
deterministic scheduling policy without interactivity causing extra preemptions
(between the workload's tasks).
Privileges and resource limits
In Linux kernels before 2.6.12, only privileged (CAP_SYS_NICE) processes can set a
non-zero static priority. The only change that an unprivileged process can make is
to set the SCHED_OTHER policy, and this can only be done if the effective user ID
of the caller of sched_setscheduler() matches the real or effective user ID of the
target process (i.e., the process specified by pid) whose policy is being changed.
Since Linux 2.6.12, the RLIMIT_RTPRIO resource limit defines a ceiling on an
unprivileged process's priority for the SCHED_RR and SCHED_FIFO policies. If an
unprivileged process has a non-zero 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 RLIMIT_RTPRIO soft limit. If the
RLIMIT_RTPRIO soft limit is 0, then the only permitted change is to lower the pri-
ority. 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 getrlimit(2) for further
information on RLIMIT_RTPRIO. Privileged (CAP_SYS_NICE) processes ignore this
limit; as with older older kernels, they can make arbitrary changes to scheduling
policy and priority.
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().
The scheduling algorithm and parameters are preserved across execve(2).
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.
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, and errno is set appropriately.
ERRORS
EINVAL The scheduling policy is not one of the recognized policies, or the parame-
ter param does not make sense for the policy.
EPERM The calling process does not have appropriate privileges.
ESRCH The process whose ID is pid could not be found.
CONFORMING TO
POSIX.1-2001. The SCHED_BATCH policy is Linux specific.
NOTES
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
getpriority(2), mlock(2), mlockall(2), munlock(2), munlockall(2), nice(2),
sched_get_priority_max(2), sched_get_priority_min(2), sched_getaffinity(2),
sched_getparam(2), sched_rr_get_interval(2), sched_setaffinity(2), sched_set-
param(2), sched_yield(2), setpriority(2), capabilities(7)
Programming for the real world - POSIX.4 by Bill O. Gallmeister, O'Reilly & Asso-
ciates, Inc., ISBN 1-56592-074-0
Linux 2.6.16 2006-03-23 SETSCHEDULER(2)
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