Raion at 23:55, 9 January 2026

2026-01-09T23:55:24Z

← Older revision		Revision as of 23:55, 9 January 2026
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	Porting: BSD nicer for nice/priority; lacks IRIX's unified interface and FRS. Real-time weaker than IRIX fixed-priority model.		Porting: BSD nicer for nice/priority; lacks IRIX's unified interface and FRS. Real-time weaker than IRIX fixed-priority model.
	Overall, IRIX scheduler reflects strong SVR4 real-time heritage with unique extensions (FRS, gang modes). illumos provides closest conceptual match; BSD covers nice semantics well but lacks advanced features. For replication, focus on vproc abstraction and capability checks. FRS is largely IRIX-specific and hard to port directly.		Overall, IRIX scheduler reflects strong SVR4 real-time heritage with unique extensions (FRS, gang modes). illumos provides closest conceptual match; BSD covers nice semantics well but lacks advanced features. For replication, focus on vproc abstraction and capability checks. FRS is largely IRIX-specific and hard to port directly.

			[[Category: Kernel Documentation]]

Raion: Created page with "== Overview == The IRIX process scheduler is accessed primarily through the schedctl(2) system call, which provides a rich set of scheduling controls reflecting IRIX's historical focus on real-time, multiprocessor, and high-performance computing workloads. It supports: Real-time fixed-priority scheduling (non-degrading) Traditional UNIX nice-value adjustments (timeshare) Process group and user-wide nice operations (BSD compatibility) CPU affinity control Scheduling mode..."

2026-01-04T03:39:46Z

Created page with "== Overview == The IRIX process scheduler is accessed primarily through the schedctl(2) system call, which provides a rich set of scheduling controls reflecting IRIX's historical focus on real-time, multiprocessor, and high-performance computing workloads. It supports: Real-time fixed-priority scheduling (non-degrading) Traditional UNIX nice-value adjustments (timeshare) Process group and user-wide nice operations (BSD compatibility) CPU affinity control Scheduling mode..."

New page

== Overview ==
The IRIX process scheduler is accessed primarily through the schedctl(2) system call, which provides a rich set of scheduling controls reflecting IRIX's historical focus on real-time, multiprocessor, and high-performance computing workloads. It supports:

Real-time fixed-priority scheduling (non-degrading)
Traditional UNIX nice-value adjustments (timeshare)
Process group and user-wide nice operations (BSD compatibility)
CPU affinity control
Scheduling mode selection (free/gang)
Full Frame Scheduler (FRS) interface for user-level hard real-time gangs
Legacy hooks for older scheduling extensions

The implementation centers on vproc_t (virtual process) objects, which abstract scheduling attributes across processes/threads. Changes are applied via VPROC_ operations that update runqueue information safely.
== Key Functions ==
Main Interface (schedctl)
Single entry point dispatching commands:

MPTS_RTPRI / MPTS_GETRTPRI: Set/get real-time priority (range NDPHIMAX..NDPLOMIN). Superuser required except for self-demotion or within normal range.
MPTS_SLICE: Set per-process time slice (ticks).
MPTS_RENICE / variants: Adjust nice values for process, process group, or all processes of a user (emulating BSD setpriority/getpriority semantics).
MPTS_SETHINTS: Initialize PRDA hints (obsolete pthread-related).
MPTS_SETMASTER: Set scheduling master PID (legacy).
MPTS_SCHEDMODE: Set scheduling mode (SGS_FREE, SGS_GANG, etc.).
MPTS_AFFINITY_*: Enable/disable/query CPU affinity.
MPTS_FRS_*: Full Frame Scheduler user interface (create, enqueue, yield, etc.).

Most operations require CAP_SCHED_MGT capability.
Process List Scanner (plistscanner / plistscan)
Utility for user-wide or group-wide operations:

Scans entire process table via procscan().
Handles renice/getnice across all matching processes.
Safe credential access via pcred_access().

== Undocumented or IRIX-Specific Interfaces and Behaviors ==
Scheduling Parameters

Real-time priorities: Fixed, non-degrading. Higher numerical value = higher priority.
NDPHIMAX (highest), NDPNORMMIN (boundary to timeshare), NDPLOMIN (lowest RT).

Nice values: Traditional UNIX (-20..+19 translated via NZERO=20).
Time slice: Configurable in ticks (default system-wide).

Scheduling Modes (via MPTS_SCHEDMODE)

SGS_FREE: Default independent scheduling.
SGS_GANG: Gang scheduling (co-scheduling of related threads).

CPU Affinity
Controlled via VPROC_SCHED_AFF; requires CAP_SCHED_MGT.
Frame Scheduler (FRS)
Hard real-time extension:

User-level creation of scheduling "frames" (gangs).
Precise control over dispatch, yield, interrupt handling.
All FRS operations require CAP_SCHED_MGT.
Old FRS interface (MPTS_OLDFRS_*) returns ENOTSUP.

Batch Job Restrictions
Processes in batch process groups (Miser) cannot be made real-time.
Permission Model
Heavy reliance on CAP_SCHED_MGT capability rather than simple uid==0 checks.
== Similarities to illumos and BSD Kernel Implementations ==
illumos (Solaris-derived)
Very close heritage (both SVR4 descendants):

schedctl-like interface existed historically in Solaris.
Real-time fixed priorities (RT class).
PRIORITY/NICE control via priocntl(2).
Gang scheduling concepts (though Solaris used processor sets).
Frame Scheduler unique to IRIX; Solaris had alternative real-time extensions.

Porting: illumos priocntl(2) system provides similar class/parameter control. FRS has no direct analog.
BSD (FreeBSD, NetBSD, OpenBSD)
More divergent:

Nice via setpriority/getpriority (exact semantics matched by IRIX MPTS_RENICE_* variants).
Real-time via sched_setparam/sched_getparam (POSIX).
CPU affinity via cpuset(2).
No direct gang or frame scheduler equivalents.
No unified schedctl; separate syscalls.

Porting: BSD nicer for nice/priority; lacks IRIX's unified interface and FRS. Real-time weaker than IRIX fixed-priority model.
Overall, IRIX scheduler reflects strong SVR4 real-time heritage with unique extensions (FRS, gang modes). illumos provides closest conceptual match; BSD covers nice semantics well but lacks advanced features. For replication, focus on vproc abstraction and capability checks. FRS is largely IRIX-specific and hard to port directly.

Kernel: Process Scheduler - Revision history

Raion at 23:55, 9 January 2026