HP-UX Reference (11i v1 00/12) - 2 System Calls (vol 5)

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rtsched(2) rtsched(2)

Each process will be controlled by an associated scheduling policy and priority. These parameters may be

speciﬁed by explicit application execution of the sched_setscheduler() or sched_setparam()

functions.

Associated with each policy is a priority range. The priority ranges for each policy can (but need not) over-

lap the priority ranges of other policies.

When a process is to be selected to run, the process that is at the head of the highest priority nonempty

process list is chosen. It is then removed from its process list.

The following scheduling policies are deﬁned:

SCHED_FIFO First in-ﬁrst out (FIFO) scheduling policy.

Processes scheduled under this policy are chosen from a process list that is ordered by

the time its processes have been in the list without being executed. Generally, the

head of the list is the process that has been in the list the longest time, and the tail is

the process that has been in the list the shortest time.

Under the SCHED_FIFO policy, the modiﬁcation of the deﬁnitional process lists is as

follows:

• When a running process becomes a preempted process, it becomes the head of

the process list for its priority.

• When a blocked process becomes a runnable process, it becomes the tail of the

process list for its priority.

• When a running process calls the sched_setscheduler()

function, the

process speciﬁed in the function call is modiﬁed to the policy and priority

speciﬁed by the param argument. If the process whose policy and priority

has been modiﬁed is a running process or is runnable, it then becomes the tail

of the process list for its new priority.

• When a running process calls the

sched_setparam()

function, the prior-

ity of the process speciﬁed in the function call is modiﬁed to the priority

speciﬁed by the param argument. If the process whose priority has been

modiﬁed is a running process or is runnable, it then becomes the tail of the

process list for its new priority.

• When a running process issues the

sched_yield()

function, the process

becomes the tail of the process list for its priority.

• At no other time is the position of a process with this scheduling policy within

the process lists affected.

For this policy, valid priorities are within the range returned by the functions

sched_get_priority_max() and sched_get_priority_min() when

SCHED_FIFO is provided as the parameter. The priority range for this policy con-

tains at least 32 priorities.

SCHED_RR Round-robin scheduling policy, with a per-system time slice (time quantum).

This policy is identical to the SCHED_FIFO policy with the additional condition that

when the implementation detects that a running process has been executing as a run-

ning process for a time period of length returned by the function

sched_rr_get_interval()

, or longer, the process becomes the tail of its pro-

cess list, and the head of that process list is removed and made a running process.

The effect of this policy is to ensure that if there are multiple

SCHED_RR processes at

the same priority, one of them will not monopolize the processor. An application

should not rely only on the use of SCHED_RR to ensure application progress among

multiple processes if the application includes processes using the SCHED_FIFO policy

at the same or higher priority levels, or SCHED_RR processes at a higher priority

level.

A process under this policy that is preempted and subsequently resumes execution as

a running process completes the unexpired portion of its round-robin interval time

period.

Section 2−−268 − 4 − HP-UX Release 11i: December 2000

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