GSP
Quick Navigator

Search Site

Unix VPS
A - Starter
B - Basic
C - Preferred
D - Commercial
MPS - Dedicated
Previous VPSs
* Sign Up! *

Support
Contact Us
Online Help
Handbooks
Domain Status
Man Pages

FAQ
Virtual Servers
Pricing
Billing
Technical

Network
Facilities
Connectivity
Topology Map

Miscellaneous
Server Agreement
Year 2038
Credits
 

USA Flag

 

 

Man Pages


Manual Reference Pages  -  EVENTTIMERS (4)

NAME

eventtimers - kernel event timers subsystem

CONTENTS

Synopsis
Description
See Also

SYNOPSIS

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.

DESCRIPTION

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.

SEE ALSO

apic(4), atrtc(4), attimer(4), hpet(4), timecounters(4), eventtimers(9)
Search for    or go to Top of page |  Section 4 |  Main Index


Powered by GSP Visit the GSP FreeBSD Man Page Interface.
Output converted with manServer 1.07.