[ntp:questions] Re: ntpq times out if NMEA refclock configured?

Richard B. Gilbert rgilbert88 at comcast.net
Sun May 14 14:23:42 UTC 2006


R Jenkins wrote:
> "Richard B. Gilbert" <rgilbert88 at comcast.net> wrote in message 
> news:L7udnQ5X_9grBvvZRVn-vA at comcast.com...
> 
>>R Jenkins wrote:
>>
>>
>>>Hi,
>>>
>>>I'm trying to add a GPS refclock to my server.
>>>After total failure with a basic Trimble TSIP output GPS plus the parse 
>>>clock, I'm now using a Garmin GPS25 and the NMEA refclock.
>>>
<big snip>
>>
>>After rereading a little more carefully, I notice that your frequency 
>>correct of -495.9 PPM is on the ragged edge of the 500 PPM limit.  It is 
>>unusual for a clock to have a freqency error this large; most are below 50 
>>PPM in absolute value.
>>
>>Does your system have a kernel parameter called "HZ"?  Is it set to a 
>>value greater than 100?  I believe I have seen references to values of 
>>both 250 and 1000; neither value works well with NTPD.  The system seems 
>>to lose clock interrupts when HZ is greater than 100.  YMMV but if you are 
>>not using 100, give it a try.
>>
> 
> Hi,
> thanks for the replies.
> 
> The -495.9 ppm seems to be a symptom of the refclock problem. Without the 
> NMEA refclock it was -60 after a few minutes, long before it had settled 
> properly.
> I think it does have a fast Hz setting (I've seen it somewhere but I can't 
> remember where or what it was set to..) However, it's a 3.2GHz processor so 
> I don't think it should struggle too much.
> 
> 
> I have the PPS pulse set to 200mS.
> The PC does not normally have a display, I use telnet (well, SSH) from my 
> desk.
> Running minicom at 4800 Baud with NTPD stopped shows the GPS serial data is 
> present:
> $GPRMC,073153,A,5319.0516,N,00106.9355,W,000.0,000.0,140506,004.0,W*76
> $GPRMC,073154,A,5319.0516,N,00106.9355,W,000.0,000.0,140506,004.0,W*71
> $GPRMC,073155,A,5319.0516,N,00106.9355,W,000.0,000.0,140506,004.0,W*70
> ...
> I'm not sure how to remotely monitor the DCD line.
> 
> 
> Simply having the 'server 127.127.20.0 prefer' line in causes the ntpq hang.
> 
> I've just got around to checking the log immediately after starting ntpd:
> 
> May 14 08:19:51 gate2 ntpd[28723]: ntpd 4.2.0a at 1.1190-r Sat May 13 10:39:48 
> BST 2006 (1)
> May 14 08:19:51 gate2 ntpd[28723]: precision = 1.000 usec
> May 14 08:19:51 gate2 ntpd[28723]: Listening on interface wildcard, 
> 0.0.0.0#123
> May 14 08:19:51 gate2 ntpd[28723]: Listening on interface wildcard, ::#123
> May 14 08:19:51 gate2 ntpd[28723]: Listening on interface lo, 127.0.0.1#123
> May 14 08:19:51 gate2 ntpd[28723]: Listening on interface eth0, 
> 192.168.0.43#123
> <Other interfaces trimmed>
> May 14 08:19:51 gate2 ntpd[28723]: kernel time sync status 0040
> May 14 08:19:51 gate2 ntpd[28723]: refclock_nmea: time_pps_kcbind failed: 
> Invalid argument
> May 14 08:19:52 gate2 ntpd[28723]: too many recvbufs allocated (40)
> 
> It looks like there is some problem with the kernel PPS interface, but I 
> have no idea what...
> I used this patch:
> PPSkit-light-alpha-3328m-2.6.15.1.diff
> on a clean download of kernel 2.6.16.9 - there were a couple of rejects, but 
> they seemed to be pretty obvious & went in easily by hand..
> 
> I'm happy to try another (recent) 2.6 kernel if there is one with a known 
> working patch?
> 
> Another test: Leaving the 'flag 3 1' out stops the refclock error line in 
> the log.
> The 'too many recvbufs allocated (40)' line seems to be triggered by the 
> NMEA refclock regardless of any other settings; it does not appear when the 
> NMEA clock is commented out in ntp.conf
> 
> Robert Jenkins.
> 
> 


If the HZ setting is causing the problem, it has little to do with 
processor speed!!   The problem seems to be that various device drivers 
mask or disable interrupts for a period covering two or more clock 
interrupts causing one or more to be lost with each occurrence.

The messages about "too many recvbufs allocated (40)" were associated 
with a bug in ntpd that I believe was fixed more than a year ago.  You 
might want to try the latest version of ntpd.  You can download it from
http://ntp.isc.org/bin/view/Main/SoftwareDownloads




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