[ntp:questions] Reference clock driver for /dev/rtc

David L. Mills mills at udel.edu
Wed Jun 23 02:41:59 UTC 2010


Pavel,

It's not as simple as that. Normally, ntpd uses settimeofday() once per 
hour to set the system clock, which has the side effect of setting the 
RTC. Obviously, you don't want that. If the RTC refclock is enabled, 
that has to be disabled, so some kind of interlock must be devised. This 
can be a tricky business and have unintended consequences if something 
or other fails. The interlocks with the PPS signal come to mind.

You are correct in that the RTC has in general far better temperature 
compensation than either the system clock or the TSC/PCC counter. 
However, its resolution is generally far worse. Even so, the lowpass 
character of the clock discipline masks this so actual delivered system 
time should be quite good. Chapter 15 of my new book due in September 
contains an extensive discussion on these issues.

Dave

Krejci, Pavel wrote:

>Hi,
>
>well, then, do you find it useful? How should I proceed to contribute into ntpd project?
>
>Thanks
>Pavel
>
>  
>
>>-----Original Message-----
>>From: unruh [mailto:unruh at wormhole.physics.ubc.ca]
>>Sent: Thursday, June 17, 2010 11:48 AM
>>To: questions at lists.ntp.org
>>Subject: Re: [ntp:questions] Reference clock driver for /dev/rtc
>>
>>On 2010-06-16, Krejci, Pavel
>><pavel.krejci at siemens-enterprise.com> wrote:
>>    
>>
>>>Hi,
>>>
>>>      
>>>
>>>>-----Original Message-----
>>>>From: unruh [mailto:unruh at wormhole.physics.ubc.ca]
>>>>Sent: Tuesday, June 15, 2010 7:15 PM
>>>>To: questions at lists.ntp.org
>>>>Subject: Re: [ntp:questions] Reference clock driver for /dev/rtc
>>>>
>>>>On 2010-06-15, Krejci, Pavel
>>>><pavel.krejci at siemens-enterprise.com> wrote:
>>>>        
>>>>
>>>>>Hi,
>>>>>
>>>>>since I cannot use kvm-clock as the clock source (older
>>>>>          
>>>>>
>>>>guest kernel) I am using pit as the clock source. According to my
>>>>tests this is the most stable clock source among tsc,hpet
>>>>        
>>>>
>>but still
>>    
>>
>>>>can drift. Since the qemu keeps the /dev/rtc perfectly
>>>>        
>>>>
>>synchronized
>>    
>>
>>>>with the Host's system time it is a good time source for
>>>>        
>>>>
>>the ntpd on
>>    
>>
>>>>the guest. The host itself is then sychronized via NTP with the
>>>>external time server. I don't know any other way how to read the
>>>>system time from the Host, please offer if you have some.
>>>>        
>>>>
>>>>I do not understand. If you driver can read the rtc, it
>>>>        
>>>>
>>can read the
>>    
>>
>>>>system clock instead.
>>>>        
>>>>
>>>I am not reading the Host's /dev/rtc. I am reading the
>>>      
>>>
>>Guest's /dev/rtc, which is synchronized with the Host's system clock.
>>    
>>
>>OK, if that is the way your virtual system works, (Ie it
>>delivers the system time via /dev/rtc) then so be it. I would
>>say it is terrible, since it uses a predefined item ( rtc) to
>>deliver something totally different ( the system time of the
>>underlying host) rtc has numberous idiosyncracies, not oleast
>>being that it delivers only times with one second precision.
>>It also delivers an interrupt on one second boundaries, is
>>written by a displacement of .5 sec (Ie if you write the time
>>x to it, that time refers to the time of the rtc .5 sec in
>>the future. ) I assume that your /dev/rtc does not have all
>>thoese peculiarities.
>>
>>
>>    
>>
>>>>And virtual systems are terrible things to use ntpd on.
>>>>        
>>>>
>>ntpd cannot
>>    
>>
>>>>control something where the clock varies by more than 500PPM, and
>>>>virtual systems, since they are shut down for variable lengths of
>>>>time by the host, tend to have terrible clocks.
>>>>        
>>>>
>>>Yes the clocks like hpet or tsc are drifting very very much
>>>      
>>>
>>and the ntpd cannot improve the resulting stability. But the
>>pit keeps quite well. With additional ntpd the resulting long
>>period clock stability is good - no exact measures done yet...
>>
>>OK. Not sure what the pit refers to in the case of the virtual system.
>>
>>    
>>
>>>>The rtc can only be read in one second chunks.
>>>>        
>>>>
>>>This does not matter, some radio clocks allow the same.
>>>
>>>      
>>>
>>>>>The only disadvantage is that when the step time back must
>>>>>          
>>>>>
>>>>be done on the Host, the /dev/rtc gets stuck (it is a monotonic
>>>>clock) and the qemu must be restarted.
>>>>
>>>>rtc is not a monotonic clock. It can be set to whatever time you
>>>>want.
>>>>Unless your hardware is different than what I am imagining.
>>>>        
>>>>
>>>This comes from the implementation of the /dev/rtc by the
>>>      
>>>
>>qemu. I haven't investigated why.
>>    
>>
>>>Regards
>>>Pavel
>>>
>>>      
>>>
>>>>>Regards
>>>>>Pavel
>>>>>
>>>>>          
>>>>>
>>>>>>-----Original Message-----
>>>>>>From: unruh [mailto:unruh at wormhole.physics.ubc.ca]
>>>>>>Sent: Tuesday, June 15, 2010 2:23 AM
>>>>>>To: questions at lists.ntp.org
>>>>>>Subject: Re: [ntp:questions] Reference clock driver for /dev/rtc
>>>>>>
>>>>>>On 2010-06-14, Krejci, Pavel
>>>>>><pavel.krejci at siemens-enterprise.com> wrote:
>>>>>>            
>>>>>>
>>>>>>>Hello,
>>>>>>>
>>>>>>>I have written the reference clock driver for /dev/rtc on
>>>>>>>              
>>>>>>>
>>>>>>Linux. We use it to synchronize the guest Linux system
>>>>>>            
>>>>>>
>>>>running in the
>>>>        
>>>>
>>>>>>qemu with the Host clock. If this is useful to someone
>>>>>>            
>>>>>>
>>>>else I would
>>>>        
>>>>
>>>>>>like to contribute to the NTP project.
>>>>>>How should I proceed?
>>>>>>            
>>>>>>
>>>>>>Why would you want to do that? The rtc is almost certainly
>>>>>>            
>>>>>>
>>>>worse than
>>>>        
>>>>
>>>>>>the system clock. Why not have the guest just read the
>>>>>>            
>>>>>>
>>>>host's system
>>>>        
>>>>
>>>>>>clock, rather than the rtc.
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>            
>>>>>>
>>>>
>>>>        
>>>>
>>
>>    
>>
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