Date: Mon, 07 Nov 2005 20:09:14 +0100 From: martinko <martinkov@pobox.sk> To: freebsd-stable@freebsd.org Subject: Re: timecounter and Hz quality in kern RELENG_6 Message-ID: <dko8oq$fuo$1@sea.gmane.org> In-Reply-To: <200511071610.jA7GAKSB047437@lurza.secnetix.de> References: <1dbad3150511070339k15a99854r@mail.gmail.com> <200511071610.jA7GAKSB047437@lurza.secnetix.de>
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Oliver Fromme wrote:
> Michael Schuh <michael.schuh@gmail.com> wrote:
> > i be very surprised about the performance of RELENG_6.
> > Congratulations to the entire Team for this very good work.
> >
> > Now i have 2 Machines installed with 6.0-RC1, and i have seen that on
> > both machines the Hz is differntly with GENERIC-Kernel.
>
> "sysctl kern.clockrate" tells you the HZ value.
> In FreeBSD 6 the dafult is 1000, unless you change
> it via "options HZ=x" in your kernel configuration.
>
> The values from systat(1) or vmstat(8) are not reliable,
> because the counters are only 32bit and can overflow.
> For example, one machine here with HZ=1000 reports only
> 428 in "vmstat -i":
>
> $ sysctl kern.boottime ; date +%s ; vmstat -i | grep clk
> kern.boottime: { sec = 1123867316, usec = 744735 } Fri Aug 12 19:21:56 2005
> 1131378875
> clk irq0 3216967596 428
>
> Dividing the counter value by the uptime (in seconds)
> seems to confirm the bogus rate of 428:
>
> $ runtime='( 1131378875 - 1123867316 )'
> $ echo '3216967596 / $runtime' | bc
> 428
>
> But the 32bit counter has already overflowed once, so
> we have to add 2^32. This gives the correct value:
>
> $ echo '( 3216967596 + 2 ^ 32 ) / $runtime' | bc
> 1000
>
> > After digging in the source i have found that timec.c have an routine for
> > computing the so called "Hz quality".
>
> During boot, the kernel probes several time counters and
> assigns "quality" values. Typically you have three of
> them (i8254, ACPI, TPC). The time counter with the
> highest quality value will be used for timing by default,
> but you can change it via sysctl if you know what you are
> doing. Type "sysctl kern.timecounter" and see the result.
are those quality values preset (i.e. TSC = 800) or are they computed
(during boot) somehow? and if the latter, how pls??
>
> > Can anyone explain me the "mystics" behind Hz quality,
> > and why or how this quality is computed and what are the
> > efforts?
>
> The reason for that is to have a time counter that is as
> precise and reliable as possible. For example, TPC has
> issues on SMP and power-managed machines, therefore it is
> not as reliable as ACPI, so usually the ACPI timecounter
> has higher quality (although it takes more clock cycles
> to query it).
>
> Oh, there's also a timecounter called "dummy", which does
> not count time at all. :-) It exists for debugging
> purposes only, AFAIK, and has a negative quality value,
> so it is never selected automatically.
>
> Best regards
> Oliver
>
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