(r2) RadioT4TimingTenMs < Cosmo

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---+  Can ntp be used to keep time accurate to within 10ms ?

---++ Initial analysis

   * It seems that a default debian/etch install: *aptitude install ntp ntpdate* gives a good default configuration that indirectly contacts a large pool of "stratum 1" ntp servers, e.g. icm.edu.pl which seems to have an atomic clock, and that the precision should be sufficient
      * *ignore* ntp-server, ntp-simple
      * optionally install ntp-doc 
   * By installing ntp, the daemon _ntpd_ will automatically be started. Do not try to run ntpdate by hand or with cron, since it should be disabled anyway, since the daemon is running. 
      * There *is* a daily cron job installed with the daemon package (ntp), but all it does is rotate the log files (instead of logrotate :P). You *can edit* /etc/cron.daily/ntp in order to keep a much longer backlog of logs - the default is just the last 7 days.
   * The typical polling interval of remote ntp servers appears to be about 15-20 minutes on 3 different machines.
   * The absolute offsets were a little less than 10ms for the last 7 days on all 3 machines.
   * The ntp algorithm in general seems to be quite intelligent, using heuristic models to overcome various real-time problems such as occasional network congestion, individual server failure, and the hardware characteristics of localhost.

   * 2008.10.20 to 2008.10.28 on simulation machine: ntp_offset_20081020_20081028_bell.png: <img src="%ATTACHURLPATH%/ntp_offset_20081020_20081028_bell.png" alt="ntp_offset_20081020_20081028_bell.png" width='570' height='570' style="float:left" />



---++ possible parameters to play with
---+++ config files
   * /etc/defaults/ntp
      * Maybe add -N  _To the extent permitted by the operating system, run the ntpd at the highest priority._ ?
   * /etc/ntp.conf
      * minpoll, maxpoll - increase frequency of "polling" servers?
         * replace _server [0123].debian.pool.ntp.org iburst_ by _server [0123].debian.pool.ntp.org iburst minpoll 6 maxpoll 8_  = 1 to 4 minutes instead of the default 1 to 17 minutes ?
            * initial 5 hours of trying *minpoll 6 maxpoll 8* suggest that this pushes offsets down to about 1ms:  <img src="%ATTACHURLPATH%/ntp_peerstats_offset_20081029_5h_maxpoll8.png" alt="ntp_peerstats_offset_20081029_5h_maxpoll8.png" width='570' height='570' style="float:left" />

---+++ source files - recompile
   * include/ntp.h
      * Clock filter algorithm tuning parameters ?
      * Selection algorithm tuning parameters ?
         * decrease: #define MINDISPERSE     .005    /* min dispersion increment */  ?

---++ TODO

---+++ Does this 10ms limit remain valid during intense simulated telescope usage?
   * Monitor this!

---+++ Check that all is OK after aptitude update/upgrade


-- Main.BoudRoukema - 28 Oct 2008

Topic attachments
I	Attachment	Action	Size	Date	Who	Comment
png	ntp_offset_20081020_20081028_bell.png	manage	2.0 K	28 Oct 2008 - 22:56	BoudRoukema
png	ntp_peerstats_offset_20081029_5h_maxpoll8.png	manage	1.0 K	29 Oct 2008 - 15:51	BoudRoukema	this is just a very short sample, it should be replaced later on

Topic revision: r2 - 29 Oct 2008, BoudRoukema

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