On Mon, Oct 17, 2011 at 10:45 AM, unruh <[email protected]> wrote:
> On 2011-10-17, Chris Albertson <[email protected]> wrote: > > 2011/10/17 Miguel Gon?alves <[email protected]> > > > >> what are the best (meaning well supported) refclocks today to > >> set up a new server? > > > > > > Likely the GPS with the best PPS is the new Oncore. But for NTP you > don't > > need 5 nanosecond accuracy. From NTP's point of view 5 nS is not better > > than 50 nS. > > In fact it is really hard for a computer to keep to 1us accuracy because > of the delays in interrupt processing. The computer has to recognize the > interrupt, deliver the notification that an interrupt has occured to the > interrupt processing driver, the driver has to then ask the operating > system for the local time to timestamp the interrupt. All that takes > time, and it is hard to keep that under 1us. The interrupt processor in the kernel is very simple and fast. I think less then a dozen lines of code are executed and none of them go off and make a secondary request or do anything that requires much time. Once you get into the interrupt handler the only thing it does is (1) load a hardware counter to memory and (2) set a bit to indicate #1 is done. Nothing is logged to disk and no secondary OS calls are made. The logging happens in a user level routine that notices the set bit then reads the stored copy of the counter But still you are right, this works at the uS level. I think getting to the handler is the problem. Sometimes the OS has to disable interrupts. And then always the state has to be saved. In other words I think the "bottle neck" is interrupt latency, not the processing that occurs inside the handler. Chris Albertson Redondo Beach, California _______________________________________________ questions mailing list [email protected] http://lists.ntp.org/listinfo/questions
