Re: [chrony-dev] Chrony "Watchdog"

[Harald Krammer <Harald.Krammer@xxxxxx>]

Sorry for my late answer. I was on vacation and my mailbox was overloaded.

Am 2011-06-10 12:43, schrieb Miroslav Lichvar:
> On Fri, Jun 10, 2011 at 09:55:59AM +0200, Harald Krammer wrote:
>> Hello *,
>> my tests are nearly completed with 1.25. One thing I have noted. 
>> If a time jump will happen into the past (done by  settimeofday) 
>> then Chrony  does not update the information's anymore. 
>> The leap in time happens by a software fault in my application. 
>> I fixed that, but I have an uneasy feeling. It is clear to me 
>> that a time-jump is a sensitive subject.
> 
> chrony currently doesn't handle external jumps in system time. On
> a large forward jump all timers will fire at once, that's not that
> bad, but on a backward jump all timer activity will stop until the
> clock is back at the time they were originally scheduled.
> 
>> Currently I uses the 'System time' from command tracking to find 
>> out the current offset. That isn't enough. A watchdog 
>> functionality may be helpful.  What do you think? 
> 
> You mean something using alarm()? What exactly would it do to recover
> from the backward jump?
> 
> One of the items in my todo list is to rework the timer facility to be
> able to use the CLOCK_MONOTONIC clock. That should solve the problem
> with external jumps nicely.
> 
> In the meantime I think it should be good enough to compare timestamp
> captured after every select() call with the previous one and if the
> difference is negative, adjust all timers by that amount. I'll look
> into it.
> 

You are right. CLOCK_MONOTONIC clock would be the best solution. In the
past I monitored the chrony process from my application. This solution
is not generally valid.

I saw you made a patch for that and it's already released in Chrony
1.26.  so I will test it.

Thank you!
Nice greetings,
Harald

-- 

Harald Krammer

Mobil +43.(0) 664. 130 59 58
Mail: Harald.Krammer (at) hkr.at

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