On Fri, Jan 04, 2008 at 10:28:33AM -0700, M. Warner Losh wrote: > In message: <[EMAIL PROTECTED]>
> I have no experience with gige networks, but the best I've been able > to do on 100baseT networks is 50us. I'm unsure what <1us > synchronization really means, since that's starting to get below the > level of system calls on fast machines. My experience in the late 90s with syncing Linux was based on using serial port 1 PPS interrupts with custom code in the kernel to optimize the serial port modem interrupt path. Time stamps of this interrupt versus the hardware based microclock (based on an incrementing counter in the CPU) were used to drive the NTP PLL which computed the frequency error and time offset for the kernel PLL. I found the microclock readings of time of day on the 1 PPS were usually within 1 us (there were a few late ones due to interrupt off intervals of course). This was on fairly slow server class hardware (Pentium Pro at 200 MHz). On ethernet I would not expect that level of accuracy due to the inherent CSMA/CD algorithm jitter... plus of course whatever the bridge in the star hub introduces... Obviously once one gets into the microsecond area one gets into the world of software relativity where what one means by time of day depends on where one is looking. What I was using to judge performance was what an interrupt level call to microclock returned... but inside a larger software system this will obviously be skewed by the various calling delays to get to the kernel... -- Dave Emery N1PRE/AE, [EMAIL PROTECTED] DIE Consulting, Weston, Mass 02493 "An empty zombie mind with a forlorn barely readable weatherbeaten 'For Rent' sign still vainly flapping outside on the weed encrusted pole - in celebration of what could have been, but wasn't and is not to be now either." _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.