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SCHED_4BSD(9) FreeBSD Kernel Developer's Manual SCHED_4BSD(9) NAME sched_4bsd - The 4.4BSD thread scheduler SYNOPSIS #include <sys/sched.h> void resetpriority(lwp_t *l); void sched_tick(struct cpu_info *ci); void sched_schedclock(lwp_t *l); void sched_pstats_hook(struct proc *p, int minslp); void sched_setrunnable(lwp_t *l); void updatepri(lwp_t *l); DESCRIPTION The traditional 4.4BSD scheduler employs a "multilevel feedback queues" algorithm, favouring interactive, short-running threads to CPU-bound ones. resetpriority() recomputes the priority of a thread running in user mode. If the resulting priority is higher than that of the current thread, a reschedule is arranged. sched_tick() gets called from hardclock(9) every 100ms to force a switch between equal priority threads. The priority of the current thread is adjusted through sched_schedclock(). The priority of a thread gets worse as it accumulates CPU time. sched_pstats_hook() gets called from sched_pstats() every Hz ticks in order to recompute the priorities of all threads. sched_setrunnable() checks if an LWP has slept for more than one second. If so, its priority is updated by updatepri(). EXAMPLES To determine the scheduler currently in use $ sysctl kern.sched.name kern.sched.name = 4.4BSD CODE REFERENCES The 4.4BSD scheduler subsystem is implemented within the file sys/kern/sched_4bsd.c. SEE ALSO csf(9), hardclock(9), mi_switch(9), sched_m2(9), userret(9) Marshall Kirk McKusick, Keith Bostic, Michael J. Karels, and John S. Quarterman, The Design and Implementation of the 4.4BSD Operating System, Addison Wesley, 1996. FreeBSD 14.1-RELEASE-p8 April 9, 2019 FreeBSD 14.1-RELEASE-p8