> Le 16 févr. 2017 à 04:09, MLewis <[email protected]> a écrit :
>
> On 15/02/2017 1:17 PM, Chris Albertson wrote:
>> Why set up a dedicated NTP server if you only have two computers that will
>> use it? ... You could save some money and just run NTP on the two computers.
>> ... NTP is almost zero load on the CPU and the best thing is the NTP
>> accuracy is not effected by CPU load…
This is not strictly true in all scenarios as the NTP thread has to be able to
get to a cpu to be able to do its thing. Higher priority, or CPU intensive
threads can starve it.
Here is the result of a little test on a 700MHz clocked 4 core uP running linux
with usual utilities NTP, cron whatever, but no apps . No priority or core
dedication implemented.
The uP is running NTP with two GPS sync’d servers at stratum 1 on the LAN plus
5 stratum 2 pool servers. poll time 64 secs for all.
1. Check the clock offset of the DUT as reported by ntpdate -d with the DUT
idle.
mike@muon /usr/home/mike $ ntpdate -d rasp3b1 2> /dev/null | grep adjust
16 Feb 11:17:46 ntpdate[11566]: adjust time server 192.168.1.157 offset
-0.000086 sec
16 Feb 11:19:32 ntpdate[11569]: adjust time server 192.168.1.157 offset
-0.000085 sec
16 Feb 11:21:18 ntpdate[11587]: adjust time server 192.168.1.157 offset
-0.000082 sec
16 Feb 11:23:05 ntpdate[11590]: adjust time server 192.168.1.157 offset
-0.000054 sec
16 Feb 11:24:51 ntpdate[11593]: adjust time server 192.168.1.157 offset
-0.000028 sec
16 Feb 11:26:37 ntpdate[11611]: adjust time server 192.168.1.157 offset
0.000008 sec
16 Feb 11:28:24 ntpdate[11614]: adjust time server 192.168.1.157 offset
0.000026 sec
16 Feb 11:30:10 ntpdate[11632]: adjust time server 192.168.1.157 offset
0.000059 sec
2. Start up 4 cpu soaker threads - in this case calculating pi to 10000 places.
11:31:00 4 cpu soakers started on rasp3b1
3. Continue checking clock offsets.
16 Feb 11:33:42 ntpdate[11638]: adjust time server 192.168.1.157 offset
-0.000089 sec
16 Feb 11:35:29 ntpdate[11656]: adjust time server 192.168.1.157 offset
-0.000235 sec
16 Feb 11:37:15 ntpdate[11659]: adjust time server 192.168.1.157 offset
-0.000393 sec
16 Feb 11:39:01 ntpdate[11662]: adjust time server 192.168.1.157 offset
-0.000512 sec
16 Feb 11:40:48 ntpdate[11680]: adjust time server 192.168.1.157 offset
-0.000547 sec
16 Feb 11:42:34 ntpdate[11683]: adjust time server 192.168.1.157 offset
-0.000492 sec
16 Feb 11:44:20 ntpdate[11686]: adjust time server 192.168.1.157 offset
-0.000438 sec
16 Feb 11:46:07 ntpdate[11704]: adjust time server 192.168.1.157 offset
-0.000397 sec
16 Feb 11:47:53 ntpdate[11709]: adjust time server 192.168.1.157 offset
-0.000393 sec
16 Feb 11:49:39 ntpdate[11712]: adjust time server 192.168.1.157 offset
-0.000357 sec
16 Feb 11:51:26 ntpdate[11730]: adjust time server 192.168.1.157 offset
-0.000206 sec
As you can see the reported clock offset increases to a max 0,5ms due to the
load on the DUT. That is within the OPs limit so he should be ok but for others
that may be too much of a hit.
4. wait till the processes stop
They all ended normally at Thu 16 Feb 12:04:36 UTC 2017
5. While continuing to check the offsets as reported by ntpdate
16 Feb 12:00:17 ntpdate[11775]: adjust time server 192.168.1.157 offset
0.000153 sec
16 Feb 12:02:03 ntpdate[11778]: adjust time server 192.168.1.157 offset
0.000188 sec
16 Feb 12:03:50 ntpdate[11781]: adjust time server 192.168.1.157 offset
0.000203 sec
16 Feb 12:05:36 ntpdate[11799]: adjust time server 192.168.1.157 offset
0.000126 sec
16 Feb 12:07:22 ntpdate[11802]: adjust time server 192.168.1.157 offset
0.000092 sec
16 Feb 12:09:09 ntpdate[11805]: adjust time server 192.168.1.157 offset
0.000096 sec
16 Feb 12:10:55 ntpdate[11823]: adjust time server 192.168.1.157 offset
0.000051 sec
16 Feb 12:12:41 ntpdate[11826]: adjust time server 192.168.1.157 offset
0.000008 sec
16 Feb 12:14:28 ntpdate[11829]: adjust time server 192.168.1.157 offset
0.000002 sec
16 Feb 12:16:14 ntpdate[11847]: adjust time server 192.168.1.157 offset
-0.000016 sec
16 Feb 12:18:00 ntpdate[11852]: adjust time server 192.168.1.157 offset
0.000007 sec
16 Feb 12:19:46 ntpdate[11855]: adjust time server 192.168.1.157 offset
0.000009 sec
16 Feb 12:21:33 ntpdate[11873]: adjust time server 192.168.1.157 offset
0.000012 sec
back to normal status.
The test is not supposed to be an all inclusive and YMMV.
There are probably methods, such a configuring detected cores for NTP ,
increasing its priority, and maybe increasing the poll interval that can
mitigate the effect.
I’ll try that to see what I get.
>
> To be able to move forward with my original application:
> By going to a separate box running NTP and a GPS reference, I will have a
> reference time that is entirely independent from whatever is going on with my
> working box. With microsecond accuracy and precision, it will be more than
> sufficient for my needs. With a dedicated ethernet connection between the
> working box and the NTP box, NTP on the working box should be able to have
> system time within 1 ms of that reference. If it's observed that isn't
> happening, then I'll remove NTP from the working box and I already have code
> than can monitor the system time against the NTP box and reset it every time
> it lags more than 1 ms. Brute and crude, but it will work for keeping my
> application within 1ms.
Toi should be ok if you see a similar profile.
Just to note that on my DUT prior to the 100% load the clock drift as reported
by NTP was -6,5ppm . Without NTP or other disciplining a 1ms offset would have
been reached in 108 secs.
>
> Or, so I think...
> I've been surprised and changed direction enough times since I headed down
> this time rabbit-hole.
>
> Michael
>
> _______________________________________________
> time-nuts mailing list -- [email protected]
> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there.
"The power of accurate observation is commonly called cynicism by those who
have not got it. »
George Bernard Shaw
_______________________________________________
time-nuts mailing list -- [email protected]
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
