Pavel,
It's not as simple as that. Normally, ntpd uses settimeofday() once per
hour to set the system clock, which has the side effect of setting the
RTC. Obviously, you don't want that. If the RTC refclock is enabled,
that has to be disabled, so some kind of interlock must be devised. This
can be a tricky business and have unintended consequences if something
or other fails. The interlocks with the PPS signal come to mind.
You are correct in that the RTC has in general far better temperature
compensation than either the system clock or the TSC/PCC counter.
However, its resolution is generally far worse. Even so, the lowpass
character of the clock discipline masks this so actual delivered system
time should be quite good. Chapter 15 of my new book due in September
contains an extensive discussion on these issues.
Dave
Krejci, Pavel wrote:
Hi,
well, then, do you find it useful? How should I proceed to contribute into ntpd
project?
Thanks
Pavel
-----Original Message-----
From: unruh [mailto:un...@wormhole.physics.ubc.ca]
Sent: Thursday, June 17, 2010 11:48 AM
To: questions@lists.ntp.org
Subject: Re: [ntp:questions] Reference clock driver for /dev/rtc
On 2010-06-16, Krejci, Pavel
<pavel.kre...@siemens-enterprise.com> wrote:
Hi,
-----Original Message-----
From: unruh [mailto:un...@wormhole.physics.ubc.ca]
Sent: Tuesday, June 15, 2010 7:15 PM
To: questions@lists.ntp.org
Subject: Re: [ntp:questions] Reference clock driver for /dev/rtc
On 2010-06-15, Krejci, Pavel
<pavel.kre...@siemens-enterprise.com> wrote:
Hi,
since I cannot use kvm-clock as the clock source (older
guest kernel) I am using pit as the clock source. According to my
tests this is the most stable clock source among tsc,hpet
but still
can drift. Since the qemu keeps the /dev/rtc perfectly
synchronized
with the Host's system time it is a good time source for
the ntpd on
the guest. The host itself is then sychronized via NTP with the
external time server. I don't know any other way how to read the
system time from the Host, please offer if you have some.
I do not understand. If you driver can read the rtc, it
can read the
system clock instead.
I am not reading the Host's /dev/rtc. I am reading the
Guest's /dev/rtc, which is synchronized with the Host's system clock.
OK, if that is the way your virtual system works, (Ie it
delivers the system time via /dev/rtc) then so be it. I would
say it is terrible, since it uses a predefined item ( rtc) to
deliver something totally different ( the system time of the
underlying host) rtc has numberous idiosyncracies, not oleast
being that it delivers only times with one second precision.
It also delivers an interrupt on one second boundaries, is
written by a displacement of .5 sec (Ie if you write the time
x to it, that time refers to the time of the rtc .5 sec in
the future. ) I assume that your /dev/rtc does not have all
thoese peculiarities.
And virtual systems are terrible things to use ntpd on.
ntpd cannot
control something where the clock varies by more than 500PPM, and
virtual systems, since they are shut down for variable lengths of
time by the host, tend to have terrible clocks.
Yes the clocks like hpet or tsc are drifting very very much
and the ntpd cannot improve the resulting stability. But the
pit keeps quite well. With additional ntpd the resulting long
period clock stability is good - no exact measures done yet...
OK. Not sure what the pit refers to in the case of the virtual system.
The rtc can only be read in one second chunks.
This does not matter, some radio clocks allow the same.
The only disadvantage is that when the step time back must
be done on the Host, the /dev/rtc gets stuck (it is a monotonic
clock) and the qemu must be restarted.
rtc is not a monotonic clock. It can be set to whatever time you
want.
Unless your hardware is different than what I am imagining.
This comes from the implementation of the /dev/rtc by the
qemu. I haven't investigated why.
Regards
Pavel
Regards
Pavel
-----Original Message-----
From: unruh [mailto:un...@wormhole.physics.ubc.ca]
Sent: Tuesday, June 15, 2010 2:23 AM
To: questions@lists.ntp.org
Subject: Re: [ntp:questions] Reference clock driver for /dev/rtc
On 2010-06-14, Krejci, Pavel
<pavel.kre...@siemens-enterprise.com> wrote:
Hello,
I have written the reference clock driver for /dev/rtc on
Linux. We use it to synchronize the guest Linux system
running in the
qemu with the Host clock. If this is useful to someone
else I would
like to contribute to the NTP project.
How should I proceed?
Why would you want to do that? The rtc is almost certainly
worse than
the system clock. Why not have the guest just read the
host's system
clock, rather than the rtc.
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