Hi list,
We'd like to achieve a sub-30 μs window of transmission accuracy across multiple devices for simulcast purposes. Ideally, sub-10 μs.
The system uses NTP and 1PPS, which works nearly all the time. We tested both NTP and chrony over a weekend, and the results were not what we expected. In the first plot, NTP+1PPS never drifted above 17 μs. In the second plot, chrony+1PPS drifted up to 281 μs:
Something is wrong with the test, the configuration, or most likely both.
The test data was generated by running the following program:
// SOF
#include <stdio.h>
#include <stdlib.h>
#include <sys/timex.h>
#include <unistd.h>
int main( void ) {
struct timex t = { 0 };
while( 1 ) {
const int clockState = adjtimex( &t );
printf( "%ld,%ld,%ld,%ld\n", t.esterror, t.maxerror, t.offset, t.jitter );
fflush( stdout );
sleep( 1 );
}
return 0;
}
// EOF
The configuration file for the chrony-side of the test follows:
# SOF
server 10.10.10..200 minpoll 0 maxpoll 0 maxdelay 0.5 iburst
# Location of ID/key pairs for NTP authentication.
keyfile /etc/chrony/chrony.keys
# Stores new gain/loss rate, for compensating the system clock upon restart.
driftfile /var/lib/chrony/chrony.drift
logdir /var/log/chrony
# Stop bad estimates from upsetting the machine clock.
maxupdateskew 100.0
# Enables kernel synchronisation (every 11 minutes) of the real-time clock.
rtcsync
# Step the system clock instead of slewing it if the adjustment is larger than
# one second, but only in the first three clock updates.
makestep 1 3
# Enable hardware timestamping of NTP packets for all interfaces.
hwtimestamp *
# Synchronize time using the rising edge of a 1-pulse-per-second clock.
refclock PPS /dev/pps1 refid PPS1 poll 0 precision 1e-9 prefer
# EOF
When running chronyc sources -v, we see:
MS Name/IP address Stratum Poll Reach LastRx Last sample
===============================================================================
#* PPS1 0 0 377 1 -502ns[ -534ns] +/- 1000ns
^- 10.10.10.200 1 0 377 0 +33us[ +33us] +/- 1186us
The plots were generated using R:
ntp.data <- read.csv( 'ntp-drift.csv', header=TRUE );
ntp.data$id <- 1:nrow(ntp.data);
plot(x=ntp.data$id,ntp.data$est.err, xlab="ntp", ylab="estimated error (us)", type="l");
chrony.data <- read.csv( 'chrony-drift.csv', header=TRUE );
chrony.data$id <- 1:nrow(chrony.data);
plot(x=chrony.data$id,chrony.data$est.err, xlab="chrony", ylab="estimated error (us)", type="l");
I'm wondering, in no particular order:
- Do the esterror values from calling adjtimex() yield an apples-to-apples comparison between chrony and NTP?
- Do we need to combine PPS1 with the NTP server?
- What else would we need to do to achieve sub-30 μs clock drift (or sub-10 μs)?
- How can we verify that the clock isn't drifting more than 30 μs, programatically (i..e., what API calls return the recent clock drift adjustment value)?
- What API call returns the most recent clock drift adjustment value in nanoseconds?
Any help is much appreciated.
