sched_setscheduler() - Unix, Linux System Call
Advertisements
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 *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 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_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
execution.
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 processes 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 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 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 setpriority(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-interactive,
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 workloads 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 processs
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 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
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 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
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
Tag | Description |
EINVAL |
The scheduling policy is not one of the recognized policies,
or the parameter 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 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.
Linuxs 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, thats 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
Programming for the real world - POSIX.4 by Bill O. Gallmeister, OReilly & Associates, Inc., ISBN 1-56592-074-0
Advertisements
|
To Continue Learning Please Login
Login with Google