Serge Bets <[EMAIL PROTECTED]> writes: > Hello Ulrich, > > On Friday, May 2, 2008 at 16:16:53 +0200, Ulrich Windl wrote: > >> Bill Unruh <[EMAIL PROTECTED]> writes: >>>>> Real men don't want the eleven-minutes mode. > > The words you attribute to Bill are mine.
You'll have to count the '>' characters... > > >> IMHO the idea to update the RTC during shutdown is broken, because if >> the system crashes, the RTC time may be wrong. > > The RTC has been good the last time it was written, but has drifted > since then. Only hwclock can compensate this drift at best. Eleven-mode > and kernel initialisation alone can't. How does hwclock know when the RTC was updated last? > > Under normal conditions without crash, the hwclock method clearly wins, > by very very far. It permits to reboot and stay at some tens of > microseconds of UTC; halt for the night and stay at some milliseconds... > Eleven-minutes mode does almost 1000 times less good. IMHO exchanging systemtime with the RTC by a user-program is broken. > > In case of crash, the hwclock method still wins, though much less > clearly. It compensates the drift since the previous clean shutdown, > counting that the RTC drifted at a fixed rate. But the rate really > can vary. That's why a good evaluation of the rate is so important. And > that's why an hwclock-in-cron is a good optional addition to > hwclock-in-shutdown. Did you consider that on Multi-Boot systems a different OS might have run that also updates the RTC? In these cases hwclock kills the correct time. > > Conclusion: crash or no-crash, with cron or without, instant reboot or > halt for the night, a well used hwclock always wins. hwclock is unnecessary if NTP is used and the kernel handles the RTC properly (IMHO). > > >> Likewise the concept of a user program getting the system time from >> the RTC during boot is broken. The kernel needs correct time as early >> as possible. > > The kernel initialisation already reads the RTC once at startup. Hwclock > rereads the RTC a little later, in the startup scripts. Hwclock is > extremely more accurate at that. And hwclock compensates drift; the > kernel doesn't. I may be stupid, but how does hwclock know the drift? Asuming it has exclusive ownership of the RTC? > > So what you say is very true. But is not an argument against hwclock; > It is an argument to call hwclock at the earliest. > > > Please note that I'm talking about the latest hwclock 2.32 from BJH, > whose accuracy has been considerably improved across the years. The > version forked for util-linux is far behind, somewhere between 200 and > 500 times less good. > > Also note that the eleven-minutes mode and hwclock are incompatible: > the former perturbates the later. To use hwclock well, you have to > disable the eleven-minutes mode. My opinion still is to disable hwclock and fix the kernel. Sorry. Regards, Ulrich _______________________________________________ questions mailing list [email protected] https://lists.ntp.org/mailman/listinfo/questions
