Date: Mon, 7 Nov 2005 17:10:20 +0100 (CET) From: Oliver Fromme <olli@lurza.secnetix.de> To: freebsd-stable@FreeBSD.ORG Subject: Re: timecounter and Hz quality in kern RELENG_6 Message-ID: <200511071610.jA7GAKSB047437@lurza.secnetix.de> In-Reply-To: <1dbad3150511070339k15a99854r@mail.gmail.com>
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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.
> 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
--
Oliver Fromme, secnetix GmbH & Co. KG, Marktplatz 29, 85567 Grafing
Dienstleistungen mit Schwerpunkt FreeBSD: http://www.secnetix.de/bsd
Any opinions expressed in this message may be personal to the author
and may not necessarily reflect the opinions of secnetix in any way.
"When your hammer is C++, everything begins to look like a thumb."
-- Steve Haflich, in comp.lang.c++
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