eventtimers —
kernel event timers subsystem
Kernel uses several types of time-related devices, such as: real time clocks,
time counters and event timers. Real time clocks responsible for tracking real
world time, mostly when system is down. Time counters are responsible for
generation of monotonically increasing timestamps for precise uptime tracking
purposes, when 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.
Kernel uses time-based events for many different purposes: scheduling,
statistics, time keeping, profiling and many other things, based on
callout(9) mechanism. These purposes now grouped
into three main callbacks:
-
hardclock()
-
callout(9) and timekeeping
events entry. Called with frequency defined by
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 unified way to control that hardware, and
to use it, supplying kernel with all required time-based events.
Each driver implementing event timers, registers them at the subsystem. It is
possible to see the list of present event timers, like this, via
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
where:
-
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.
Timers management code of the kernel chooses one timer from that list. Current
choice can be read and affected via
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 currently
selected time counter to precisely schedule all needed events and programs
event timer to generate 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.
apic(4),
atrtc(4),
attimer(4),
hpet(4),
timecounters(4),
eventtimers(9)