Scheduling Attributes

This section describes the AIX POSIX 1003.1c implementation-specific support, behavior, and restrictions for thread scheduling attributes.

Start Policy

The start policy attribute is fully supported by the AIX implementation of the Threads Module and defaults to RW_THR_START_RUNNING.

Contention Scope

In POSIX 1003.1c-compliant systems, support for contention scope is optional. Even if the API indicates that priority scheduling is supported, the environment may only allow one policy.

The AIX implementation of POSIX 1003.1c supports both process and system scope threads. The Threads Module maps its RWContentionScope values to the underlying POSIX 1003.1c API as follows:

Table 138. AIX: Mapping of RWContentionScope to POSIX 1003.1c values

Threads Module RWContentionScope

POSIX 1003.1c Contention Scope

RW_THR_PROCESS_SCOPE

PTHREAD_SCOPE_PROCESS

RW_THR_SYSTEM_SCOPE

PTHREAD_SCOPE_SYSTEM

Scheduling Inheritance Policy

The scheduling inheritance policy attribute is fully supported by the AIX implementation of the Threads Module and defaults to RW_THR_INHERIT.

Concurrency Policy

The concurrency policy attribute is not supported in the AIX implementation of the Threads Module. Attempts to get or set this attribute value will result in exceptions.

Scheduling Policy

In POSIX 1003.1c-compliant systems, support for scheduling policy is optional. The Threads Module determines whether scheduling policy is supported by testing for the definition of the macro _POSIX_THREAD_PRIORITY_SCHEDULING.

The AIX implementation of the Threads Module supports all three scheduling policies as defined by the standard:

SCHED_FIFO -- Specifies FIFO Scheduling, where threads run until preempted by a thread of higher priority, or until blocked. Thread priorities are set by the application; the system does not dynamically change a thread's priority.

SCHED_RR -- Selects round-robin scheduling, where the highest-priority threads runs until preempted by a thread of higher priority, until some time-quantum has elapsed, or until blocked. Threads possessing the same priority value get time-sliced and scheduled in a round-robin fashion. Thread priorities are set by the application; the system does not dynamically change a thread's priority.

SCHED_OTHER -- Selects the default scheduling policy for an implementation. This policy typically uses time-slicing with dynamic adjustments to priority and or time-slice quantum.

Note that the AIX POSIX implementation limits the scheduling policies SCHED_RR and SCHED_FIFO to processes with superuser privileges. Therefore, the RWSchedulingPolicy values RW_THR_PREEMPTIVE and RW_THR_TIME_SLICED_FIXED are limited to superusers as well.

Table 139 shows how the Threads Module AIX POSIX implementation maps RWSchedulingPolicy values to the underlying POSIX 1003.1c policy values.

Table 139. AIX: Mapping of RWSchedulingPolicy to POSIX 1003.1c values

Threads Module RWSchedulingPolicy Values

POSIX 1003.1c Scheduling Policy

RW_THR_PREEMPTIVE

SCHED_FIFO

RW_THR_TIME_SLICED_FIXED

SCHED_RR

RW_THR_TIME_SLICED_DYNAMIC(RW_THR_OTHER may also be used to set)

SCHED_OTHER

Attempts to set any other policy values result in an RWTHROperationNotAvailable exception. None of these policies may be explicitly requested unless the process has superuser privileges.

Note that the Threads Module has mapped two policy values to the same underlying policy, SCHED_OTHER. Calls to getSchedulingPolicy() return RW_THR_TIME_SLICED_DYNAMIC because that value gives the most meaningful interpretation.

Scheduling Priority

In POSIX 1003.1c-compliant systems, support for the specification of thread priority is optional. The Threads Module determines whether scheduling priority is supported by testing for the definition of the macro _POSIX_THREAD_PRIORITY_SCHEDULING.

The AIX POSIX implementation does support priority scheduling. The Threads Module uses the standard POSIX.1b (formerly POSIX.4) functions sched_get_priority_min() and sched_get_priority_max() to determine the legal range of priority values.

Under this implementation, the priorities vary according to the scheduling policy, as shown in Table 140.

Table 140. AIX with POSIX 1003.1c: Scheduling policy priority values

Scheduling Policy

Minimum Priority

Maximum Priority

Default Priority

RW_THR_PREEMPTIVE

1

127

1

RW_THR_TIME_SLICED_FIXED

1

127

1

RW_THR_TIME_SLICED_DYNAMIC

1

127

1

A new thread's priority value is inherited from the creating thread by default, unless the priority attribute has been explicitly set, or the inheritance policy has been changed from its default value of RW_THR_INHERIT to RW_THR_EXPLICIT. If the inheritance policy is RW_THR_EXPLICIT, the Threads Module defines the default priority to be the minimum priority.

The AIX POSIX threads implementation ignores attempts to set the priority if the thread scope is RW_THR_SYSTEM_SCOPE and the scheduling policy is RW_THR_TIME_SLICED_DYNAMIC. Threads with this scheduling policy are subject to continuous priority adjustments.

The allowed range of priorities on AIX is different from the priorities that are actually used. A priority in the range of 1 to 39 results in a priority of 40, and a priority in the range of 81 to 127 results in a priority of 80. So the effective range of priority values is 40 to 80. Only privileged users can set a priority greater than 60.

Scheduling Time-Slice Quantum

The time-slice quantum attribute is not supported in the AIX implementation of the Threads Module. Attempts to get or set this attribute value will result in exceptions.