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List: freebsd-hackers
Subject: Re: Timer Granularity
From: Mike Smith <mike () smith ! net ! au>
Date: 1998-10-30 5:04:46
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>
> I am confused, On basis of which parameters is the kernel clock
> granularity defined?? especially when multiple clocks are available.
At which release level? 2.1.x, 2.2.x or 3.x?
As a general rule, the best available clock is used. The techniques
used to determine which clock is best, and to ensure accurate counts
vary from release to release, with a general trend towards better.
>
> On Thu, 29 Oct 1998, Mike Smith wrote:
>
> > > I am new to the FreeBSD OS. I have a few questions for which I could not
> > > find answers on the webpages.
> > >
> > > I am using the FreeBSD version 2.1.5 Relase #2. What is the kernel clock
> > > granularity in this version?
> >
> > 10ms, if I remember correctly.
>
> >
> > > There are a number of clocks being used in FreeBSD. Which clock is used
> > > for real time kernel processing? Is this accessible to the users?
> >
> > No clocks are "accessible to the users". User applications obtain time
> > values from the system.
> >
> > > Has the timer granularity improved in the releases following this release?
> >
> > Yes. Timer support now offers nanosecond resolution, dependant on the
> > particular clock source(s) in use.
> >
> > > In the /sys/kern/kern_clock.c file the variable time_precision has been
> > > initialised to 1microsec, but it has been stated that the resolution
> > > decreases depending on whether the external clock is working or not.
> > > What is the range of the variation in the resolutuion?
> >
> > It depends on the hardware in use. On some systems the Pentium TSC is
> > available as a time source, in which case time resolution is one CPU
> > cycle. The i8254 timecounter generally has an operating frequency
> > around 1.19MHz, giving a resolution of approximately 1us.
> >
> > --
> > \\ Sometimes you're ahead, \\ Mike Smith
> > \\ sometimes you're behind. \\ mike@smith.net.au
> > \\ The race is long, and in the \\ msmith@freebsd.org
> > \\ end it's only with yourself. \\ msmith@cdrom.com
> >
> >
>
>
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>
--
\\ Sometimes you're ahead, \\ Mike Smith
\\ sometimes you're behind. \\ mike@smith.net.au
\\ The race is long, and in the \\ msmith@freebsd.org
\\ end it's only with yourself. \\ msmith@cdrom.com
To Unsubscribe: send mail to majordomo@FreeBSD.org
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