Re: [chrony-users] Re: Locking PPS refclock with NTP source

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Well, I found out that if i run a cpu stress test (i am using mprime 'torture test') while running chrony, suddenly my PPS source std dev goes stable below 300ns.

So i guess this is realted to some kind of cpu scheduling. However i disabled the c-state options in the bios, my CPU frequency is not altered according to /proc/cpuinfo, and yet i still have bad results unless i run this stress test.

Any idea ?

2017-09-01 15:52 GMT+02:00 Miroslav Lichvar <mlichvar@xxxxxxxxxx>:
On Thu, Aug 31, 2017 at 05:25:09PM +0200, Thibaut BEYLER wrote:
> No I tested it on ordinary x64 kernel (4.9 , 3.16 etc..) coming from debian
> & fc repos with no custom options.
> I just installed pps-linux and invoked 'ldattach 18 /dev/ttyS0' (which also
> loads pps_ldisc module) , maybe i'm missing something ?

I'm not sure. Maybe the hardware cannot do better. From what I have
seen, with a real serial port (no USB, etc) the PPS is usually stable to around
1 microsecond. For example, this is what I get with a Garmin 18x I have here:

MS Name/IP address         Stratum Poll Reach LastRx Last sample
#* PPS0                          0   4   377     8   +200ns[ +391ns] +/-  281ns

Name/IP Address            NP  NR  Span  Frequency  Freq Skew  Offset  Std Dev
PPS0                       11   6   161     +0.001      0.015    +18ns   415ns

Miroslav Lichvar

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