Address: 127.127.28.u
Reference ID: SHM
Driver ID: SHM
This driver receives its reference clock info from a shared memory-segment. The shared memory-segment is created with owner-only access for unit 0 and 1, and world access for unit 2 and 3
struct shmTime {
int mode; /* 0 - if valid set
* use values,
* clear valid
* 1 - if valid set
* if count before and after read of
* values is equal,
* use values
* clear valid
*/
int count;
time_t clockTimeStampSec; /* external clock */
int clockTimeStampUSec; /* external clock */
time_t receiveTimeStampSec; /* internal clock, when external value was received */
int receiveTimeStampUSec; /* internal clock, when external value was received */
int leap;
int precision;
int nsamples;
int valid;
int dummy[10];
};
Each second, the valid-flag of the shared memory-segment is checked:
If set, the values in the record (clockTimeStampSec, clockTimeStampUSec, receiveTimeStampSec, receiveTimeStampUSec, leap, precision) are passed to ntp, and the valid-flag is cleared and a counter is bumped.
If not set, a counter is bumped
Each second, the valid-flag of the shared memory-segment is checked:
If set, the count-field of the record is remembered, and the values in the record (clockTimeStampSec, clockTimeStampUSec, receiveTimeStampSec, receiveTimeStampUSec, leap, precision) are read. Then, the remembered count is compared to the count now in the record. If both are equal, the values read from the record are passed to ntp. If they differ, another process has modified the record while it was read out (was not able to produce this case), and failure is reported to ntp. The valid flag is cleared and a counter is bumped.
If not set, a counter is bumped
The gpsd man page suggests setting minpoll and maxpoll to 4. That was an attempt to reduce jitter. The SHM driver was fixed (ntp-4.2.5p138) to collect data each second rather than once per polling interval so that suggestion is no longer reasonable.
The 4th field is the number of second ticks since the last poll. The 5th field is the number of good data samples found. The last 64 will be used by ntpd. The 6th field is the number of sample that didn't have valid data ready. The 7th field is the number of bad samples. The 8th field is the number of times the the mode 1 info was update while nptd was trying to grab a sample.
Here is a sample showing the GPS reception fading out:
54364 84927.157 127.127.28.0 66 65 1 0 0 54364 84990.161 127.127.28.0 63 63 0 0 0 54364 85053.160 127.127.28.0 63 63 0 0 0 54364 85116.159 127.127.28.0 63 62 1 0 0 54364 85180.158 127.127.28.0 64 63 1 0 0 54364 85246.161 127.127.28.0 66 66 0 0 0 54364 85312.157 127.127.28.0 66 50 16 0 0 54364 85375.160 127.127.28.0 63 41 22 0 0 54364 85439.155 127.127.28.0 64 64 0 0 0 54364 85505.158 127.127.28.0 66 36 30 0 0 54364 85569.157 127.127.28.0 64 0 64 0 0 54364 85635.157 127.127.28.0 66 0 66 0 0 54364 85700.160 127.127.28.0 65 0 65 0 0