[ntp:questions] "ntpd -q" is slow compared to ntpdate

Steve Kostecke kostecke at ntp.org
Tue Oct 14 20:19:00 UTC 2008


On 2008-10-14, Mohit Aron <extproxy at gmail.com> wrote:

> I've ready in numerous places that ntpdate is going to be deprecated
> and that one should use 'ntpd -q -g' instead. I have also read
> complaints by people that 'ntpd -q -g' is slow, but I haven't read
> about any suitable resolution to this issue. My own system uses 'ntpd
> -q -g' to synchronize with an Internal time server and the call to
> 'ntpd -q -g' takes more than 3 minutes to complete.

In my tests 'ntpd -gq' takes all of 11 seconds to run with an ntp.conf
which just specifies a time server on the LAN.

>This slows up the startup of my machine.

If you don't need to block the boot process while the clock is initially
set there is no need to use 'ntpq -gq'. Starting ntpd with '-g' is
enough to ensure that the time will be set correctly no matter how far
off the system clock may be.

> Here's the /etc/ntp.conf of my client machine (where the our local ntp
> server has the IP 10.50.33.100):

Reordered for clarity ...

> logfile /var/log/ntp.log
> driftfile /var/log/ntp.drift
> restrict default notrust nomodify notrap noquery
>
> server 10.50.33.100 iburst minpoll 4
> restrict 10.50.33.100
>
> server 127.127.1.0
> fudge 127.127.1.0 stratum 11
> restrict 127.127.1.0

The Undisciplined Local Clock (127.127.1.x) is not needed unless this
ntpd is serving time to others. However this has nothing to do with your
slow start-up.

> I also ran a tcpdump to capture the ntp packets being exchanged between the
> client and the server. It seems the client sends a total of 13 requests to
> the server - each of which is responded to immediately. The first 9 requests
> are spaced at period of 2 seconds each. The 10th one is sent one second
> after the 9th request. The 11th one is sent 18s after then 10th. The 12th
> one is sent 65 seconds after the 11th. Finally, the 13th one is sent 66s
> after the 12th one. Personally, I would have been extremely happy if 'ntpd
> -q -g' terminated after the first request was sent and the reply was
> received.

ntpd has to collect a certain amount of data before it will believe a
particular time source. The first volley of polls are generally more
than enough to allow ntpd to determine a close approximation of the
correct time. If the clock is more than 128ms off it will be stepped
otherwise a slew will be initiated; ntpd exits immediately after the
step or the initiation of the slew.

> Issuing 'ntpdate 10.50.33.100' completes almost instantaneously.

That's because ntpdate blindly accepts the first reply it receives.

-- 
Steve Kostecke <kostecke at ntp.org>
NTP Public Services Project - http://support.ntp.org/




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