[LinuxPPS] hardpps: PPSJITTER: jitter=80945, limit=67744

[Alexander Gordeev]

В Fri, 24 Dec 2010 11:22:07 +0100
Udo van den Heuvel <udovdh at xs4all.nl> пишет:

> Alexander,
> 
> On 2010-12-24 03:00, Alexander Gordeev wrote:
> > Udo van den Heuvel <udovdh at xs4all.nl> пишет:
> > 
> >> I see messages like
> >>
> >> hardpps: PPSJITTER: jitter=80945, limit=67744
> >>
> >> every few minutes in my messages file. How come? How to fix?
> > 
> > I suspect that you have CONFIG_NO_HZ enabled.
> 
> There is no software fix to make NO_HZ behave like e.g. 100HZ?

Try this patch as a workaround:

diff --git a/include/linux/timex.h b/include/linux/timex.h
index d23999f..f89117e 100644
--- a/include/linux/timex.h
+++ b/include/linux/timex.h
@@ -259,7 +259,11 @@ static inline int ntp_synced(void)
 
 #define NTP_SCALE_SHIFT		32
 
+#ifdef CONFIG_NO_HZ
+#define NTP_INTERVAL_FREQ  (2)
+#else
 #define NTP_INTERVAL_FREQ  (HZ)
+#endif
 #define NTP_INTERVAL_LENGTH (NSEC_PER_SEC/NTP_INTERVAL_FREQ)
 
 /* Returns how long ticks are at present, in ns / 2^NTP_SCALE_SHIFT. */


> >> Do I need RT kernel?
> > 
> > Should not be necessary.
> 
> Well, I would like to combine the power saving of NO_HZ with good
> timekeeping. I know that they can be hard to combine, but this is just
> catching an interrupt every now and then?

There is no problem catching an interrupt. The problem is to
synchronize to it.

> > Can you please try the latest patchset:
> > https://github.com/downloads/ago/linux-2.6/patch-2.6.37-rc6-ts22.bz2
> > 
> > Kconfig magic in it ensures that users never select both CONFIG_NO_HZ
> > and CONFIG_NTP_PPS at the same time. Please note that CONFIG_NTP_PPS is
> > not selected automatically now so you may have to select it manually.
> 
> As soon as 2.6.37 hits post-rc status I will give it a go!
> (firewall is ntp server, needs stable kernel)

I think 2.6.37 will be out pretty soon.

-- 
  Alexander
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