[PlanetCCRMA] NTP anyone?

Raymond rmartin@colba.net
Fri Jul 25 11:34:01 2003

Florin Andrei wrote:

>Anyone noticed any anomalies when doing media stuff and running ntpd?
>Normally, ntpd is not supposed to make the system time jump, but
>sometimes it does. For example, on some bad systems, if you do things
>like CD-writing, for some reason the system CPU usage goes through the
>roof. The system is very unresponsive meanwhile (no matter which
>kernel), and afterwards you end up with a system clock that's quite a
>few seconds behind.
>Shortly thereafter, ntpd makes the clock jump.
>I also suspect that there are other situation when ntpd makes the system
>clock do "micro-jumps".
>Whatever the case, that sometimes made my video players run amok: the
>video becomes very jerky, the sound confused and hissing, etc. A simple
>restart of the application is enough to restore it.
>So, i wonder if anyone else noticed bad things happening when running
>ntpd on systems that depend heavily on a precise and uniform clock.
NTP continuously makes the system time jump when the clock is not correctly in synch with a time server, albeit in very small ncrements. The time increments are slewed to bring the local ntpd in sync with another reference time server. In extreme cases of a clock way out of time then ntpd with set the time instead of slewing. AFAIK, this is normal behavior for NTP.

If you are CD-writing on a bad system it is no surprise that it is unresponsive and that NTP will set the clock shortly thereafter.

With respect to the video player. It sounds like that is the problem.
The video player is just checking the system time in order to operate instead of having its own internal reference based on time independent of the clock program. It could be badly designed.

If you do not have problems with other programs that involve at least some soft real-time operation your video program is the likely cause of the problem.

One fix is to reduce the update frequency of your local NTP time server 
with an external one. On a personal system you should not need to 
synchronize with the external time server more than once(or a couple of 
times) a day. The most clock drift that you should see within a day is 
on the order of a few seconds(aside form the CPU cycle sucking you 

Anyway, unless you definitely need NTP, do not use it. Most people do 
not need NTP even when they are doing audio and video. If you are trying 
to synchronize applications distributed across networked systems or keep 
accurate time records(day, hour, minute, second) of events then you need 

Make it simple and not much simpler.

Raymond: Martin