[LinuxPPS] ntptime status - relaunch of that issue

[Felix Joussein]

Hello Michael,

I have checked that the current clock source is tsc and that 3 different
are available in total: tsc, acpi_pm and jiffies

I have changed them to see what happens:

tsc & acpi_pm show no differences...
jiffies let's the beeper beep permanently... as soon as the keyboard
produces a short beep (for ex. backspace on begin of line)... and after
setting jiffies you can't reset it to tsc (the echo tsc > /sys/.....) is
useless... not exewcuted, because a cat of the file still shows jiffies.

I also have compared the behavior  from my former 2.4 installation
mentioned in my posts before, and realized, that the ntpd would not even
start.
In dmesg I get: many lines "do_clock_gettime: negative time wrap on
CPU#0: -60751978ns (0,7646704)" and ntpd produces
"adjtimex: ntpd could be using obsolete ADJ_TICKADJ" and then stops
itself again.

The ppstest equivalent in 2.4 is ppsctl, the output is pretty similar to
ppstest, the jitter is bouncing from 640 to -32, -672, 704 per line.

when I try to run ntpd -d I get a lot debugging output, and at the end a
freezed screen. (So definitely this hardware s***ks, maybe I better turn
it into a DVR device!)

The current machines, running 2.4 kernel + PPSKit are Pentium III with
512MB Ram...

The Idea to use Pentium M's was to limit power usage.... the industrial
PC I'm using now has a Pentium M and 2 GB RAM is using 80W at most...
So: how about an Intel ATOM?
So, what would be the recommended hardware setup?

thank's for helping me!

regards,

Felix Joussein


Michael Meier schrieb:
>> I played around with different hardware and realised, that then faster a
>> machine is, then less the offset / jitter is.
>> For example on an amd64 X2 5000+ jitter/offset is about 0,100 - 0,010
>> +/- while with the Pentium M processor the offset/jitter rises up far
>> beyond 1,000...
>> I think, the Pentium M (1700MHz) should be way enough for a ntp server?
>
> The CPU Power surely is enough, but you might hit an architectural
> problem with the Pentium M: It has a timestamp counter [1] that does
> NOT increase monotonically, even when all power saving features are
> disabled. Since the TSC is the default and preferred timesource of the
> kernel, that will cause some confusion. The kernel will detect that
> the TSC is crap after some time, and switch to another source after
> some time if it has some available, but the TSC really is the best you
> can get. The only thing that comes close to it is HPET (High Precision
> Event Timer), and that is not available everywhere. And naturally, if
> ntpd cannot get the local time from the kernel with a sufficiently
> high resolution, things will start to jitter.
> You should check what your kernel currently uses as timesource (cat
> /sys/devices/system/clocksource/clocksource0/current_clocksource), and
> what sources it has available
> (/sys/devices/system/clocksource/clocksource0/current_clocksource).
> You can also check in the kernel logs, the kernel usually tells you
> when it changes the timesource and why.
>
> [1] http://en.wikipedia.org/wiki/Time_Stamp_Counter