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mdoc and language improvements.

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MFC after:	1 week
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Christian Brueffer 2014-04-12 21:04:53 +00:00
parent 4787115d04
commit a59ff25642
Notes: svn2git 2020-12-20 02:59:44 +00:00 
svn path=/head/; revision=264384
1 changed files with 18 additions and 17 deletions
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35
share/man/man4/timecounters.4
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@ -31,20 +31,20 @@
.Nm timecounters
.Nd kernel time counters subsystem
.Sh SYNOPSIS
Kernel uses several types of time-related devices, such as: real time clocks,
The 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
Real time clocks are responsible for tracking real world time, mostly when the system
is down.
Time counters are responsible for tracking purposes, when system is running.
Event timers are responsible for generating interrupts at specified time or
Time counters are responsible for tracking purposes, when the system is running.
Event timers are responsible for generating interrupts at a specified time or
periodically, to run different time-based events.
This page is about the second.
.Sh DESCRIPTION
Time counters are the lowest level of time tracking in kernel.
Time counters are the lowest level of time tracking in the kernel.
They provide monotonically increasing timestamps with known width and
update frequency.
They can overflow, drift, etc and so in raw form used only in very limited
performance-critical places like process scheduler.
They can overflow, drift, etc and so in raw form can be used only in very limited
performance-critical places like the process scheduler.
.Pp
More usable time is created by scaling the values read from the selected
time counter and combining it with some offset, regularly updated by
@ -54,13 +54,14 @@ on
invocation.
.Pp
Different platforms provide different kinds of timer hardware.
The goal of the time counters subsystem is to provide unified way to access
The goal of the time counters subsystem is to provide a unified way to access
that hardware.
.Pp
Each driver implementing time counters, registers them at the subsystem.
It is possible to see the list of present time counters, like this, via
Each driver implementing time counters registers them with the subsystem.
It is possible to see the list of present time counters, via the
.Va kern.timecounter
sysctl:
.Xr sysctl 8
variable:
.Bd -literal
kern.timecounter.choice: TSC-low(-100) HPET(950) i8254(0) ACPI-fast(900) dummy(-1000000)
kern.timecounter.tc.ACPI-fast.mask: 16777215
@ -81,7 +82,7 @@ kern.timecounter.tc.TSC-low.frequency: 11458556
kern.timecounter.tc.TSC-low.quality: -100
.Ed
.Pp
where:
The output nodes are defined as follows:
.Bl -inset
.It Va kern.timecounter.tc. Ns Ar X Ns Va .mask
is a bitmask, defining valid counter bits,
@ -90,13 +91,13 @@ is a present counter value,
.It Va kern.timecounter.tc. Ns Ar X Ns Va .frequency
is a counter update frequency,
.It Va kern.timecounter.tc. Ns Ar X Ns Va .quality
is an integral value, defining how good is this time counter,
comparing to others.
Negative value means that this time counter is broken and should not be used.
is an integral value, defining the quality of this time counter
compared to others.
A negative value means this time counter is broken and should not be used.
.El
.Pp
Time management code of the kernel chooses one time counter from that list.
Current choice can be read and affected via
The time management code of the kernel chooses one time counter from that list.
The current choice can be read and affected via the
.Va kern.timecounter.hardware
tunable/sysctl.
.Sh SEE ALSO