>> Is there any point to it without the matching kernel driver? > >Has anybody tried asking for the "echo" from user space? > >That is: > grab time > raise modem signal > grab time > read timestamp over serial port > >If you precede that with > grab time > lower modem signal >I think that will warm up most of the cache. Another interesting experiment >would be to see how much of a difference that makes.
The trimble driver already uses user-space for its time mark output (the HW_poll function). It works quite well: on my test machine (50ns precision, ovenized oscilator) a Praecis Cf has a jitter of less than 30ns, even with a 40k packets-per-second load. I don't remember pre-fetching a timestamp making much of a difference when I was testing changes to the driver. HW_poll does not echo a PPS input, so its alignment to the top of the second is arbitrary. Attached is an ntpvis summary csv of a run I just finished on my test machine while unloaded. Unit0 is a Praecis Cf with usermode timemark, and unit1 is a Palisade using the timemark kernel module I am developing. I am making another run now with the Palisade using the usermode timemark for comparison. Currently my kernel module has the same arbitrary-alignment problem as ntpsec's HW_poll, so it's not quite ready for general use.
trimble-lkm-summary.csv
Description: Binary data
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