GNU/Linux man pages
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GNU/Linux |
RedHat 5.2(Apollo) |
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sched_setscheduler(2) |
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sched_setscheduler, sched_getscheduler − set and get scheduling algorithm/parameters
#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 {
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... | |
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int sched_priority; | |
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... |
};
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>.
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.
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ESRCH |
The process whose ID is pid could not be found. | ||
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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. | ||
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EINVAL |
The scheduling policy is not one of the recognized policies, or the parameter p does not make sense for the policy. |
POSIX.1b (formerly POSIX.4)
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.
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).
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sched_setscheduler(2) | |