Add ntpviz, and minpoll=maxpoll=0 to time service howto.

1 files changed, 29 insertions, 1 deletions

diff --git a/www/gpsd-time-service-howto.txt b/www/gpsd-time-service-howto.txt
index 414508dc..a541a7c2 100644
--- a/www/gpsd-time-service-howto.txt
+++ b/www/gpsd-time-service-howto.txt
@@ -1139,6 +1139,11 @@ used by the ntpd suite to measure and correct the system time.  It is not
 directly applicable to chronyd, although some design considerations
 may be similar.
 
+It is hard to optimize what you can't visualize.  The easiest way to
+visualize ntpd performance is with ntpviz from <<NTPSEC.ORG>>.  Once you
+are regularly graphing your server performance it is much easier to see
+the results of changes.
+
 === NTP performance tuning ===
 
 For good time stability, you should always have at least four other
@@ -1285,6 +1290,13 @@ these scripts for your particular setup.
 These scripts are for the combination of GPSD and ntpd.  If you use
 chronyd, you *will* need to modify these, at the least.
 
+==== ntpviz procedure ====
+
+If all this calculating and graphing looks painful, then grab a copy
+of ntpviz from <<NTPSEC.ORG>>. ntpviz generates lots of pretty graphs
+and html pages.  It even calculates the correct IMT offset, and other
+performance metrics for you.
+
 ===== Format of the loopstats and peerstats files =====
 
 The following is incorporated from the ntpd website, see <<NTP-MONOPT>>
@@ -1342,7 +1354,7 @@ You should also have a decent set of NTP servers you are syncing to.
 [start=1]
 . Running ntpd without IMT
 
-Locate the line in your ntp.conf that refers to the shm0 segment and
+Locate the line in your ntp.conf that refers to the SHM0 segment and
 append 'noselect' to it.  As an example, the first two lines in the sample
 above will become:
 
@@ -1399,6 +1411,18 @@ the offset between the 1PPS time and the GPS time.
 If, for example, your estimate of the offset is -0.32s, your time1 fudge
 value will be '0.32'.  Note the change of sign.
 
+=== Pollling Interval ===
+
+ntpd seems to better use a PPS refclock when the polling interval is
+as small as possible.  The ntpd default minpoll is 6, and can be set to
+as low as 4.  NTPsec versions 0.9.5 and above of ntpd allow you to
+set minpoll and maxpoll as low as 0.  Changing minpoll from 4 to 0 may
+reduce your PPS jitter by over a factor of 4.
+
+-----------------------------------------------------------------------------
+server 127.127.28.1 minpoll 0 maxpoll 0 prefer
+-----------------------------------------------------------------------------
+
 == Chrony performance tuning
 
 The easiest way to determine the offset with chronyd is probably to
@@ -1829,4 +1853,8 @@ by Jaap Winius <jwinius@rjsystems.nl>.
 2.9 August 2016
     Fix typos.
 
+2.10 September 2016
+    Mention ntpviz
+    Recommend minpoll=maxpoll=0 for PPS refclocks
+
 // end