rtsched.2 (2010 09)

ManualsBrandsHP ManualsSoftwareHP-UX Manpages

rtsched(2) rtsched(2)

PRI_HPUX_TO_POSIX(), PRI_POSIX_TO_HPUX()

These two functions serve to map (translate) the range of HP-UX priorities into the POSIX.4 model.

These translations are necessary because the POSIX.4 standard chose larger numbers to represent

stronger priorities and the existing HP-UX behavior, which must be maintained for backward compatibil-

ity, uses smaller numbers for stronger priorities.

The

PRI_HPUX_TO_POSIX()

function returns the POSIX.4 scheduling priority corresponding to the

HP-UX priority passed in the argument pri . The value of pri can be any integer.

The

PRI_POSIX_TO_HPUX()

function returns an HP-UX process priority corresponding to the priority

passed in the argument pri. The value of pri can be any integer. The HP-UX priority returned is com-

parable to the values returned by

getpriority()

(see getpriority (2)).

Scheduling Policies

The scheduling policies described are deﬁned in terms of a conceptual model, which contains a set of

thread lists. There is, conceptually, one thread list for each priority. Any runnable thread may be in any

thread list. Multiple scheduling policies are provided. Each nonempty list is ordered, and contains a

head as one end of its order, and a tail as the other. The purpose of a scheduling policy is to deﬁne the

allowable operations on this set of lists (for example, moving threads between and within lists).

Each thread 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. (However, these functions change scheduling parameters at process level for multithreaded

processes).

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

overlap the priority ranges of other policies.

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

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

The following scheduling policies are deﬁned:

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

Threads scheduled under this policy are chosen from a thread list that is ordered by

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

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

is the thread that has been in the list the shortest time.

Under the

SCHED_FIFO policy, the modiﬁcation of the deﬁnitional thread lists is

as follows:

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

the thread list for its priority.

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

thread list for its priority.

• When a running thread 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 thread of a process whose policy and

priority have been modiﬁed is running or is runnable, it then becomes the tail

of the thread list for its new priority.

• When a running thread calls the

sched_setparam() function, the priority

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

by the param argument. If the thread of a process whose priority has been

modiﬁed is running or is runnable, it then becomes the tail of the thread list

for its new priority.

• When a running thread issues the

sched_yield() function, the thread

becomes the tail of the thread list for its priority.

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

the thread 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.

4 Hewlett-Packard Company − 4 − HP-UX 11i Version 3: September 2010