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. > 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. 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. 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. Conclusion: crash or no-crash, with cron or without, instant reboot or halt for the night, a well used hwclock always wins. > 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. 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. Serge. -- Serge point Bets arobase laposte point net _______________________________________________ questions mailing list [email protected] https://lists.ntp.org/mailman/listinfo/questions
