[ntp:questions] SNTP server + ntpd 4.2.4 client

Dennis Hilberg, Jr. timekeeper at dennishilberg.com.invalid
Mon Mar 17 19:18:02 UTC 2008

Unruh wrote:
>> FreeBSD has built-in PPS support (no patch needed), but it's not enabled by 
>> default. PPS support has to be enabled in the kernel config and the kernel 
>> recompiled.
> And exactly what is that supposed to buy you? 
> You need something which can be interrupted by the pps signal from the gps,
> and read the system time on that interrupt. That does not require kernel
> support (except of course being able to respond to interrupts-- and if your
> kernel cannot do that, you forgot to switch on your computer.)

If I understand correctly, it allows the kernel to discipline the local 
clock directly from the PPS signal, instead of having a separate helper 
program like shmpps pass it to a shared memory segment for ntpd to handle, 
which will discipline the clock via the SHM driver. I don't know if one 
method is better as I've tried both and there didn't seem to be much 
difference as far as I could tell. But at the least having PPS support 
enabled allows you to use the type 20 (generic NMEA GPS receiver) driver 
which will deal with both the PPS and NMEA time. That would eliminate both 
the need to have upstream servers for time sources as well as the separate 
helper program. However, most timekeepers will (should) have a few upstream 
servers for sanity checks anyway, and most systems nowadays can easily 
handle a small driver like shmpps running all the time.


Dennis Hilberg, Jr.     \  timekeeper(at)dennishilberg(dot)com
NTP Server Information:  \  http://saturn.dennishilberg.com/ntp.php

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