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comp.protocols.time.ntp
Discussion complète de l'article :
Linux 11-minute mode (RTC update)
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Article :
27425
Date :
03-04-2008
From :
Noob
Sujet :
Linux 11-minute mode (RTC update)
Hello everyone,
I run Linux kernel 2.6.22.1-rt9 and ntpd 4.2.4p0
# adjtimex --print
mode: 0
offset: 77
frequency: -1309904
maxerror: 493576
esterror: 50
status: 1
time_constant: 6
precision: 1
tolerance: 33554432
tick: 10000
raw time: 1207230744s 183249us = 1207230744.183249
In my setup, STA_UNSYNC (0x0040, clock unsynchronized) is 0.
Thus, ntp_synced() returns 1.
Thus the kernel should write the system time to the RTC every
11 minutes; but it does not.
The relevant code is in sync_cmos_clock()
http://lxr.linux.no/linux/kernel/time/ntp.c#L188
I've added several printk() to this function, and it appears
that it is never called.
The relevant timer is defined with the following macro.
static DEFINE_TIMER(sync_cmos_timer, sync_cmos_clock, 0, 0);
which expands to
static struct timer_list sync_cmos_timer =
{
.function = sync_cmos_clock,
.expires = 0,
.data = 0,
.base = &boot_tvec_bases
};
The problem seems to be that this timer is never armed, to bootstrap
the process. It seems there should be a call to mod_timer() somewhere.
do_adjtimex() calls notify_cmos_timer() unconditionally.
static void notify_cmos_timer(void)
{
if (no_sync_cmos_clock)
mod_timer(&sync_cmos_timer, jiffies + 1);
}
What are the semantics of notify_cmos_timer?
What is it supposed to do?
And what is 'no_sync_cmos_clock' supposed to mean?
/* Disable the cmos update - used by virtualization and embedded */
int no_sync_cmos_clock __read_mostly;
Why would we (re)arm the timer when 'no_sync_cmos_clock' is true?
I'd be grateful for anyone sharing their knowledge.
Regards.
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Article :
27442
Date :
04-04-2008
From :
Serge Bets
Sujet :
Re: Linux 11-minute mode (RTC update)
Hello,
On Thursday, April 3, 2008 at 16:25:17 +0200, Noob wrote:
> STA_UNSYNC (0x0040, clock unsynchronized) is 0. [...] Thus the kernel
> should write the system time to the RTC every 11 minutes; but it does
> not.
Fine! Don't touch anything, happy man, or it might well "tomber en
marche". Real men don't want the eleven-minutes mode. It is not only
extremely inaccurate by itself, but it also steps on the toes of those
tools that are able to manage the RTC properly.
I previously posted some figures, comparing the accuracy of writing the
RTC (not counting drift). Mean offset and dispersion:
- eleven-minutes mode: -2150 µs +-5000
- hwclock util-linux-ng 2.13.1: -2500 µs +-170
- hwclock 2.32 from BJH: 0 µs +-10
Furthermore both hwclocks are able to evaluate and compensate the drift
of the RTC. The eleven-minutes mode cannot do that, and instead it can
perturbate hwclock's calculations.
Serge.
--
Serge point Bets arobase laposte point net
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Article :
27443
Date :
04-04-2008
From :
Noob
Sujet :
Re: Linux 11-minute mode (RTC update)
Serge Bets wrote:
> Noob wrote:
>
>> STA_UNSYNC (0x0040, clock unsynchronized) is 0. [...] Thus the kernel
>> should write the system time to the RTC every 11 minutes; but it does
>> not.
>
> Fine! Don't touch anything, happy man, or it might well "tomber en
> marche".
Hello Serge, I was hoping you'd comment!
If I don't want the kernel to update the RTC, I can always undef
CONFIG_GENERIC_CMOS_UPDATE.
> Real men don't want the eleven-minutes mode.
:-)
> It is not only extremely inaccurate by itself, but it also steps on
> the toes of those tools that are able to manage the RTC properly.
Our equipment is supposed to run 24/7 for months / years.
I need to keep the RTC synchronized, in case of power failure.
Do you believe that running hwclock --systohc periodically is better
than using the kernel's 11-minute mode?
> I previously posted some figures, comparing the accuracy of writing the
> RTC (not counting drift). Mean offset and dispersion:
>
> - eleven-minutes mode: -2150 µs +-5000
> - hwclock util-linux-ng 2.13.1: -2500 µs +-170
> - hwclock 2.32 from BJH: 0 µs +-10
Point taken.
( I use http://giraffe-data.com/software/about_hwclock.html )
If I use hwclock to update the RTC, how often should I do it?
What do you think about the following script?
while true
do
sleep 660 # or some other value?
hwclock --utc --systohc
done
> Furthermore both hwclocks are able to evaluate and compensate the drift
> of the RTC. The eleven-minutes mode cannot do that, and instead it can
> perturbate hwclock's calculations.
Is the crystal of the RTC supposed to be more stable than the crystal
of the CPU?
Regards.
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