SCHED_4BSD(9)

💅 SCHED_4BSD — the 4.4BSD thread scheduler ✨

okay so bestie, welcome to the sched_4bsd page from the Kernel Developer's Manual 🎀 — yes it's a section 9 manpage, yes it's about the kernel's vibes-based (no, actually very much algorithm-based, ngl) thread scheduler. buckle up 🦋

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✨ NAME ✨

sched_4bsd — the classic 4.4BSD thread scheduler, the OG main character of kernel scheduling 👑

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🌸 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);

like, that's the whole cast of characters 💖

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💖 DESCRIPTION 💖

So the traditional 4.4BSD scheduler uses a “multilevel feedback queues” algorithm — and honestly its whole personality is: it favours interactive, short-running threads over CPU-bound ones. lowkey biased but in a cute way 🫶. the chatty little threads get priority, the CPU hogs get pushed back, iykyk.

• resetpriority() — recomputes the priority of a thread that's running in user mode. and if the resulting priority is higher than that of the current thread? 👀 a reschedule gets arranged. politely. but firmly.
• sched_tick() — gets called from hardclock(9) every 100ms to force a switch between equal priority threads. it's giving referee energy 💀
• sched_schedclock() — this is where the current thread's priority gets adjusted. heads up: the priority of a thread gets worse as it accumulates CPU time. yes literally, the more CPU you eat the more the scheduler side-eyes you 🙈
• sched_pstats_hook() — gets called from sched_pstats() every Hz ticks, all so it can recompute the priorities of all the threads. group project energy, everyone gets recalculated ✨
• sched_setrunnable() — checks if an LWP has slept for more than one second 🛌. if it has, its priority gets updated by updatepri(). nap girlies stay winning 🧚

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⭐ EXAMPLES ⭐

wanna know which scheduler is currently in use? bestie just ask it:

$ sysctl kern.sched.name
kern.sched.name = 4.4BSD

slay, confirmed 4.4BSD 🔥

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🎀 CODE REFERENCES 🎀

The 4.4BSD scheduler subsystem is implemented within the file sys/kern/sched_4bsd.c — that's where the magic lives, go read it 💜

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🌈 SEE ALSO 🌈

• csf(9)
• hardclock(9)
• mi_switch(9)
• sched_m2(9)
• userret(9)

and if you're feeling academic (respect, honestly 👑): 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.

💀 fr fr if you're touching this, remember: priority isn't a personality, it changes. don't hardcode assumptions about it, bestie.

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okay that's the whole page, you ate that 💅 stay iconic, read your manpages, and remember — this little number shipped on Debian, dated April 9, 2019 🗓️🌸 bye bestie, mwah 🫶💖