EVENTTIMERS(4)

📦 version 15.1, in freebsd 14.5, freebsd 15.1, nixpkgs unstable

🦋 EVENTTIMERS(4): event timers, but make it make sense 🦋

okay so this is the EVENTTIMERS(4) page from the Kernel Interfaces Manual, and it’s literally all about the kernel’s event timers subsystem, bestie ✨

🌸 NAME 🌸

eventtimers — the kernel event timers subsystem 💅

✨ SYNOPSIS ✨

okay so the kernel uses several types of time-related devices, like: real time clocks, time counters and event timers. Real time clocks are responsible for tracking real world time, mostly when the system is down. Time counters are responsible for generation of monotonically increasing timestamps for precise uptime tracking purposes, when the system is running. Event timers are responsible for generating interrupts at specified time or periodically, to run different time-based events. This page is about the last one — event timers, obviously 🎀

💖 DESCRIPTION 💖

The kernel uses time-based events for many different purposes: scheduling, statistics, time keeping, profiling and many other things, based on the callout(9) mechanism. These purposes are now grouped into three main callbacks:

  • hardclock() — callout(9) and timekeeping events entry. Called with frequency defined by the hz variable, usually 1000Hz ⏰
  • statclock() — statistics and scheduler events entry. Called with frequency about 128Hz 📊
  • profclock() — profiler events entry. When enabled, called with frequency about 8KHz 🔥

Different platforms provide different kinds of timer hardware. The goal of the event timers subsystem is to provide a unified way to control that hardware, and to use it, supplying the kernel with all required time-based events. It’s giving “one interface to rule them all” energy 💅

Each driver implementing event timers registers them at the subsystem. It is possible to see the list of present event timers, like this, via the kern.eventtimer sysctl:

kern.eventtimer.choice: HPET(550) LAPIC(400) i8254(100) RTC(0)
kern.eventtimer.et.LAPIC.flags: 15
kern.eventtimer.et.LAPIC.frequency: 0
kern.eventtimer.et.LAPIC.quality: 400
kern.eventtimer.et.i8254.flags: 1
kern.eventtimer.et.i8254.frequency: 1193182
kern.eventtimer.et.i8254.quality: 100
kern.eventtimer.et.RTC.flags: 17
kern.eventtimer.et.RTC.frequency: 32768
kern.eventtimer.et.RTC.quality: 0
kern.eventtimer.et.HPET.flags: 7
kern.eventtimer.et.HPET.frequency: 14318180
kern.eventtimer.et.HPET.quality: 550

so what’s the tea on those values? 👀

  • kern.eventtimer.et.X.flags is a bitmask, defining event timer capabilities:
    • 1 — periodic mode supported,
    • 2 — one-shot mode supported,
    • 4 — timer is per-CPU,
    • 8 — timer may stop when CPU goes to sleep state,
    • 16 — timer supports only power-of-2 divisors.
  • kern.eventtimer.et.X.frequency is a timer base frequency,
  • kern.eventtimer.et.X.quality is an integral value, defining how good is this timer, comparing to others.

The timers management code of the kernel chooses one timer from that list. Current choice can be read and affected via the kern.eventtimer.timer tunable/sysctl. Several other tunables/sysctls are affecting how exactly this timer is used:

  • kern.eventtimer.periodic — allows to choose periodic and one-shot operation mode. In periodic mode, periodic interrupts from timer hardware are taken as the only source of time for time events. One-shot mode instead uses the currently selected time counter to precisely schedule all needed events and programs the event timer to generate an interrupt exactly in specified time. Default value depends of chosen timer capabilities, but one-shot mode is preferred, until other is forced by user or hardware.
  • kern.eventtimer.singlemul — in periodic mode, specifies how much times higher timer frequency should be, to not strictly alias hardclock() and statclock() events. Default values are 1, 2 or 4, depending on configured HZ value.
  • kern.eventtimer.idletick — makes each CPU to receive every timer interrupt independently of whether they busy or not. By default this options is disabled. If chosen timer is per-CPU and runs in periodic mode, this option has no effect — all interrupts are always generating.
bestie, if a timer is dangerous, deprecated, or behaves weirdly, the page is very clear about it — no sugarcoating, just facts with sparkle ✨

🦋 SEE ALSO 🦋

💌 and the footer on this one says Debian, dated March 13, 2012 — love that for us. bye bestie! 💖

📦 where this page comes from