On Thu, 18 Nov 2010 18:00:11 +0000, Miernik, Jerzy (Jerzy) wrote: > 2. Are offsets as small as to be measured in (tens of) nanoseconds > really achievable, say among Linux workstations on a LAN?
Not readily. Tens of microseconds is achievable. One issue is the time and time variability of timestamping by the kernel and library. I've measured typical time for execution of gettimeofday() and clock_gettime() of several microseconds on desktop systems, with variation in individual times ranging from a couple of microseconds minimum to over a hundred microseconds maximum (embedded systems compiled with special options may be able to do better). Another issue is interrupt latency (e.g. in handling incoming network packets or PPS signals). A third is the transit time for packets between systems -- e.g. with switched gigabit Ethernet, buffers in the switch ports may delay packets from one machine to another. > 3. Why would > people develop PTP (precision time protocol) while nanokernel was > available? PTP requires special hardware (NIC with hardware timestamping support) to achieve performance better than NTP. Equipment designed for PTP typically has such hardware. Without such hardware support, (i.e. PTP on general purpose hardware, NTP), synchronization is affected by software timestamping variability (see above), and PTP w/o the hardware support is comparable in performance to NTP. _______________________________________________ questions mailing list [email protected] http://lists.ntp.org/listinfo/questions
