Re: [chrony-users] Syncing time as with a single request

[Lonnie Abelbeck <lists@xxxxxxxxxxxxxxxxxxx>]

Hi Bill,

I appreciate your comments, but chronyd is called immediately after the foreground "chronyd -q ..." so that should mitigate your concerns.

Lonnie


> On Mar 1, 2020, at 4:27 PM, Bill Unruh <unruh@xxxxxxxxxxxxxx> wrote:
> 
> There is a battle between acuracy and speed. Your time might be synched, but
> your rate may be way off. Thus your clock might lose 10 sec per minute with
> your procedure. To get a good estimate for the rate you need to spend more
> time. (with 4 sec you might reduce the rate to something like 20-200PPM from
> true 1 sec per second ( I am assuming that the accuracy of your reading the
> remote clock is something like 5 to 50 microseconds error.) Now you may not
> care, which is fine, But you might, as soon as the system comes up, want to
> find out the some elapsed time to high accuracy. Then this system is not
> great. Of course if all you care is is that the the system is synced to one
> second and the rate is correct to 1% is good enough. Then that is fine. But
> you might want to go into your decisions with eyes open.
> 
> 
> 
> On Sun, 1 Mar 2020, Lonnie Abelbeck wrote:
> 
>> 
>> 
>>> On Jan 28, 2020, at 2:35 AM, Miroslav Lichvar <mlichvar@xxxxxxxxxx> wrote:
>>> 
>>> On Tue, Jan 28, 2020 at 04:11:49AM +0000, Gustav Krantz wrote:
>>>> Would it be possible to do the initial sync with a single request per server? If so how would this be done and what would the drawback be?
>>> 
>>> The current development code in git supports a single-sample
>>> selection/update mode, enabled by setting maxsample to 1, but it's
>>> meant to be used only in the "ntpdate" mode (-q/-Q option) as it
>>> doesn't adjust the frequency of the clock. You could use it for the
>>> initial correction before normally starting chronyd.
>>> 
>>> # chronyd -q 'pool pool.example.com maxsamples 1 iburst'
>>> 
>> 
>> I have been testing this new "maxsamples 1" feature with the latest chrony-master.tar.gz snapshot of 2020-02-19.
>> 
>> Works very nicely.  Our project first does a quick foreground "chronyd -q ..." step (historical from sntp/ntpdate days) and then followed by a background chronyd .
>> 
>> With "maxsamples 1", our boot times are 4-6 seconds faster.
>> 
>> Using time.cloudflare.com, the step is performed in about 230 ms.  (versus 4.4 seconds before)
>> --
>> pbx4 ~ # time chronyd -q -t 8 "server time.cloudflare.com maxsamples 1 iburst"
>> 2020-03-01T21:31:53Z chronyd version DEVELOPMENT starting (+CMDMON +NTP +REFCLOCK +RTC +PRIVDROP -SCFILTER -SIGND +ASYNCDNS -SECHASH -IPV6 -DEBUG)
>> 2020-03-01T21:31:53Z Initial frequency -17.927 ppm
>> 2020-03-01T21:31:53Z System clock wrong by -0.000330 seconds (step)
>> 2020-03-01T21:31:53Z chronyd exiting
>> 
>> real	0m0.229s
>> user	0m0.006s
>> sys	0m0.005s
>> --
>> 
>> While testing, a '-t 8' is needed since DNS errors/misconfigurations can cause "chronyd -q ..." to hang, seemingly forever.
>> 
>> Example, using a typo .con instead of .com ... Control-C'ed to quit:
>> --
>> pbx4 ~ # time chronyd -q "server time.cloudflare.con maxsamples 1 iburst"
>> 2020-03-01T21:34:23Z chronyd version DEVELOPMENT starting (+CMDMON +NTP +REFCLOCK +RTC +PRIVDROP -SCFILTER -SIGND +ASYNCDNS -SECHASH -IPV6 -DEBUG)
>> 2020-03-01T21:34:23Z Initial frequency -17.927 ppm
>> ^C2020-03-01T21:41:07Z chronyd exiting
>> 
>> real	6m43.632s
>> user	0m0.003s
>> sys	0m0.010s
>> --
>> 
>> Adding the '-t 8' guards against those special DNS issues, without it a box could hang at startup.  Example, same typo with a '-t 8':
>> --
>> pbx4 ~ # time chronyd -q -t 8 "server time.cloudflare.con maxsamples 1 iburst"
>> 2020-03-01T21:41:38Z chronyd version DEVELOPMENT starting (+CMDMON +NTP +REFCLOCK +RTC +PRIVDROP -SCFILTER -SIGND +ASYNCDNS -SECHASH -IPV6 -DEBUG)
>> 2020-03-01T21:41:38Z Initial frequency -17.927 ppm
>> 2020-03-01T21:41:46Z chronyd exiting
>> 
>> real	0m8.014s
>> user	0m0.002s
>> sys	0m0.009s
>> --
>> 
>> Lonnie
>> 
>> 
>> 
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