[ntp:questions] Linux 11-minute mode (RTC update)

Serge Bets serge.bets at NOSPAM.laposte.invalid
Mon Apr 7 16:33:01 UTC 2008

 On Sunday, April 6, 2008 at 19:36:40 +0000, Unruh wrote:

> Serge Bets <serge.bets at NOSPAM.laposte.invalid> writes:
>> The eleven-minutes write takes some tens of microseconds; normal
>> hwclock some tens of milliseconds; hwclock --nointerrupt some plain
>> seconds.
> To write the rtc properly takes at least a second.

I was talking about processor time consumed. As for wall clock time
elapsed, indeed a background cron job mostly sleeping or waiting for an
interrupt during one or two seconds is rather harmless.

> In the case the machine comes up again in a few seconds, the rate
> correction is irrelevant anyway.

Drift compensation is important even for a stop and go: maybe the stop
was a crash, and the RTC was last written hours ago. But the point was
more that as long as the downtime stays short, the temperature of the
RTC, and thus its instant drift rate, does not vary too much from the
mean runtime values. Therefore it makes sense to pick the runtime rate
for compensation. Only in this specific 24/7 case.

> And if the system reads the clock badly it will be out by about a
> second anyway.

This never happens on a proper setup: hwclock --hctosys syncs on the RTC
down to a few microseconds.

Serge point Bets arobase laposte point net

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