Hal Murray wrote: > Spoon wrote: > >> I've noticed something I find very strange on the systems I have to work >> with. Every time I reboot the computer, the clock skew of the local >> clock changes, sometimes by what seems to be a huge amount. >> >> For example, I boot the computer, let ntpd run for 12 hours, and the >> value recorded in the drift file is 35 ppm. I reboot the computer, let >> ntpd run for 12 hours, and I get 5 ppm... > > I'm chasing the same glitch.
I'm relieved to know I am not alone :-) > I've seen it on two systems, both i386 running Linux 2.6 kernel. > > I think I've tracked it to tsc_init which calls calculate_cpu_khz > both are in ./arch/i386/kernel/tsc.c Take a look at init/calibrate.c too. They use the timestamp counter to calibrate lpj (loops per jiffy). You can skip the calibration by providing lpj at boot-time. If you don't provide lpj, it is estimated, and the kernel prints: Calibrating delay using timer specific routine.. 2535.19 BogoMIPS (lpj=12675953) If you specify lpj=N on the boot command, the kernel prints: Calibrating delay loop (skipped)... 2535.16 BogoMIPS preset > tsc_init prints a line like this: > kernel: Detected 2793.226 MHz processor. > > The problem is that calculate_cpu_khz doesn't return the > same answer. I hacked the code to call/print it 10 times > and I get things like this: > kernel: Detected 2793.287 MHz processor. > kernel: Detected 2793.225 MHz processor. > kernel: Detected 2793.228 MHz processor. > kernel: Detected 2793.304 MHz processor. > kernel: Detected 2793.242 MHz processor. > kernel: Detected 2793.192 MHz processor. > kernel: Detected 2793.334 MHz processor. > kernel: Detected 2793.203 MHz processor. > kernel: Detected 2793.292 MHz processor. > kernel: Detected 2793.237 MHz processor. > > That's a spread of about 50 ppm which matches what I've seen > before I started looking for this glitch. I will definitely take a look! Regards. _______________________________________________ questions mailing list [email protected] https://lists.ntp.isc.org/mailman/listinfo/questions
