| Re: [chrony-dev] Leap second handling |
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On 07/10/12 23:49, Bill Unruh wrote:
On Tue, 10 Jul 2012, Frank Muzzulini wrote:
Hello,
sadly I was not able to observe the leap second in June and I also
found that the online documentation gives little information about how
chrony handles a leap second. I guess it forwards the information to
the kernel, doesn't it?
You may have heard about the the problems some linux systems had and
the solution implemented by Google. (If you haven't, look at
http://www.h-online.com/open/news/item/Leap-second-Linux-can-freeze-1629805.html).
I wonder if chrony could add something like that as an option, too.
Alternatively I can also imagine a simpler way: Pass the leap second
announcement to clients as usual and locally adapt the time as if it
were an error of one second, encountered at 0:00 UTC. If clients use
the same setting, it will work neatly for all anyway.
Well, my version of chrony seems to have handled it without any problem. If
you look at www.theory.physics.ubc.ca/chrony youwill see no evidence of any
problem .
I am not speaking about a problem of chrony, I want chrony to be a
solution to other problems. Let me express this more clearly.
There are actually two problems:
1. The general one: The posix interface to get the system time
(gettimeofday or clock_gettime(CLOCK_REALTIME) does not forsee any
representation of leap seconds. In practice, at least in Linux, from the
application point of view a leap second looks like a back step of one
second.
2. The specific one: Some linux kernels have chance to consume endless
CPU after passing a leap second.
I am sure the linux bug will be fixed by the time of the next leap
second, although some old kernels might still be around and new bugs
might come up. The general problem will remain.
Btw: I have no permission to look at your server, but I found that one
of our test machines was running chrony during the leap second and so I
could see that it passes the information to the kernel as expected.
--
Frank MUZZULINI Phone: +49 721 9497-2501
COMSOFT GmbH Fax: +49 721 9497-349
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