[time-nuts] NTP jitter with Linux

[Mike S]

On 4/5/2012 2:51 AM, David J Taylor wrote:

> Mike, have you tried FreeBSD instead? Does it show the same problems?

I have a couple of Soekris Net 4501s running FreeBSD and NTP. They don't 
have much jitter, but they're a very different architecture. The machine 
with the jitter is my home "do all" machine - router, firewall, 
file/email/print/web/ntp server, etc.

It's certainly not critical to get rid of the jitter, it's a time-nuts 
thing. It bothers me. I noticed that when my backup process ran, NTP 
jitter dropped very significantly. I'm sure it's related in some way to 
interrupt latency - the first thing the PPS driver does when a PPS 
interrupt comes in is to save a nanotimestamp.

Dennis Ferguson mentioned the OS idle loop. So, I added "no-hlt" to the 
kernel boot line. This did improve things quite a bit. All else the 
same, it reduces jitter to the 3-4 us range - better than the 20 us 
without, but not as good as the 1-2 when I simply load up the processor.