On Mon, Aug 01, 2016 at 02:40:11AM -0700, Hal Murray wrote: > > There are two parts to PPS processing in the kernel. RFC 2783 describes an > API for capturing time stamps. RFC 1589 describes a PLL that lives in the > kernel. > > Most Linux distros don't support RFC 1589. The code is in the kernel, but it > doesn't work with the shipped kernels. It requires !NO_HZ, but most distros > prefer NO_HZ. > > I pulled over the sources and built my own kernel. > > Here are the before and after graphs: > http://users.megapathdsl.net/~hmurray/ntpsec/PPS-kernel.png > The data is from two separate days so this isn't a clean comparison. I don't > know what that machine was doing on either day. > > Here is a zoom in on the Kernel PLL day. > http://users.megapathdsl.net/~hmurray/ntpsec/PPS-kernel2.png > Note that the peak offset is less than a microsecond. > > ------------ > > We should see if we can get similar results on a Raspberry Pi. I haven't > tried building an ARM kernel. > > I think we should be able to run the PLL code outside the kernel. The PPS > time stamp is key. The PLL calculations don't need to be run in the kernel. > They need to be run soon after the PPS, but not interrupt level immediately. > The API has an option to wakeup on PPS. I don't know if it is implemented on > Linux. > > The no-PLL test was run at the default maxpoll of 6. I should try faster. I > also need a standard test load. > > I remember various FreeBSD-is-better type comments from many years ago. I > don't know if the PLL was working in Linux at the time. I should setup a > test case.
Hey Hal, I'm using maxpoll of 1 on my stratum 1 servers. And I have !NO_HZ set. My offsets stay belong 1 microsecond as reported by ntpq. If we switched the units to nanoseconds, that might be interesting. I don't have !NO_HZ set on my stratum 2 servers, but I'm looking at the ramifications of that. I'm curious what your results are. Cheers, -Matt _______________________________________________ devel mailing list devel@ntpsec.org http://lists.ntpsec.org/mailman/listinfo/devel
