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.
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 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 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 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.
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).
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>.