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On Wed, 11 May 2016, Jahagirdar, Bhushan wrote:
Hi Miroslav,
Thanks for the reply.
My second question about limiting samples is nothing to do with the bug.
Question <1>
Let me put the question in other way. chronyd accepts "n" number of samples before it actually updates the time. How can this "n" be configured? How does chronyd decides upon how many samples it needs to come to conclusion
Chrony does a least squares fit to the data to determine the offset and rate.
Thus it needs at least 3 samples to do that. It tests whether the a linear fit
is a reasonable approximation (by looking at the number of changes in sign of
the difference of the measured offsets from the fit line seem to be randomly
distributed) and if not, it reduces the number of samples (thowing out the old
ones). Thus in the ideal case the number of samples used will grow to a max of
something like 64.This reduces the variance.
that the source data is authentic and system time should be set?
3. But chrony does not step the clock. It slews it, rapidly if necessary.
makestep introduces the danger of back steps which can have very bad effects
on the filesystem.
My assumption is makestep is one of the configuration that can make chronyd jump to the expected time. I have used "makestep 1000 -1" in the configuration so that there should not be gradual change as system time could be set to epoch. But still chronyd takes in many samples (approx 35) to set the system time.
chrony for the PPS gets one sample for every 16PPS samples (poll of 4)
throwing out about 40% of the samples which are furthest from the median to
stop "popcorn" erors from causing trouble. Thus it will take about 48 PPS
samples to collect the 3 inputs needed. I do not know if you can reduce the
poll to less than 4 (16 sec)
Question <2>
With the workaround I am able to feed data from multiple sources. Please consider below logs for my question below:
1970-01-01T00:00:04Z chronyd version DEVELOPMENT starting (+CMDMON +NTP +REFCLOCK +RTC +PRIVDROP -SCFILTER -SECHASH +ASYNCDNS +IPV6 -DEBUG)
1970-01-01T00:00:04Z commandkey directive is no longer supported
1970-01-01T00:00:04Z Could not open keyfile /etc/chrony/chrony.keys
1970-01-01T00:00:04Z Frequency 0.000 +/- 1000000.000 ppm read from /var/log/chrony/chrony.drift
1970-01-01T00:00:52Z Selected source SOC2
1970-01-01T00:00:52Z System clock wrong by 1609373625.736444 seconds, adjustment started
2020-12-31T00:14:38Z System clock was stepped by 1609373625.736444 seconds
2020-12-31T00:14:54Z Can't synchronise: no majority
What does this mean? "Can't synchronise: no majority".
chrony looks at the various sources to see if they agree about what the time
is. With 2 sources, it is really hard to get a majority. 1 or 3 sources are
needed (3 to outvote one bad source)
--
Thanks and Regards,
Bhushan J.
-----Original Message-----
From: Miroslav Lichvar [mailto:mlichvar@xxxxxxxxxx]
Sent: Wednesday, May 11, 2016 2:14 PM
To: chrony-users@xxxxxxxxxxxxxxxxxxxx
Subject: Re: [chrony-users] Multiple interfaces is possible?
On Wed, May 11, 2016 at 07:01:17AM +0000, Jahagirdar, Bhushan wrote:
Attached strace log and config file that I am using.
Chrony version I tried: 2.2.1
Thanks. In the log chronyd is trying to read the SOCK sample from a wrong file descriptor. That fails, select() returns that there are still data waiting on the right descriptor, chronyd tries to read from the wrong descriptor again, and this runs in an infinite loop.
It looks like the problem is with the reallocation of the first SOCK refclock instance. When the second SOCK instance is added, the pointer that the scheduler is using for the first instance becomes invalid.
This bug was apparently introduced in 2.0.
I'm not sure what's the best way to fix it yet. As a workaround you can add two more refclocks before the real refclocks to prevent chronyd from reallocating the third refclock instance. E.g.
refclock SOCK /var/run/chrony/chrony.dummy1.sock
refclock SOCK /var/run/chrony/chrony.dummy2.sock
refclock SOCK /var/run/chrony/chrony.ttyS1.sock refclock SOCK /var/run/chrony/chrony.ttyS2.sock
--------------------------------
Also, I would like to know how can the minimum sampling limit be set for chronyd?
I am using makestep as below with the assumption that system is set to epoch.
makestep 1000 -1
Still the number of samples being analyzed are many. How to reduce this?
I'm not sure which samples do you want to limit. Is this related to the bug described above which causes an infinite loop?
--
Miroslav Lichvar
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