[ntp:questions] Red Hat vote for chrony

William Unruh unruh at invalid.ca
Mon Dec 8 17:19:52 UTC 2014


On 2014-12-08, Charles Swiger <cswiger at mac.com> wrote:
> On Dec 7, 2014, at 7:19 PM, William Unruh <unruh at invalid.ca> wrote:
>>> I suspect most people are a bit more likely to use air conditioning to control ambient
>>> temperature changes then they are to desolder and swap out their crystals in the
>>> hopes of obtaining more precise timekeeping....
>> 
>> Actually air conditioning is largely irrelevant because it is not
>> ambient air temperature changes that are most important, but temp
>> changes inside the case caused by differing loads on the system. 
>
> Data point: for a normal desktop machine I have at home, which has a 
> 95W TDP i5 CPU and a 145W 970 GPU, I can see a ~12C temperature change
> on the motherboard temp sensor between idle and full load on both as the
> total system draw goes from ~90 W to ~350 W, if I don't use air-conditioning.
>
> With air-conditioning on, the temperature change shrinks to about 5C,
> which reduces the thermal wandering of the XO by a factor of 2.  That
> seems to be a worthwhile improvement, not "largely irrelevant".

I find that very stange. Does the temperature in the room really
oscillate by 5 degrees if you run your machine? (Ie, you ascribe 5C of
the temp change to the room). 

>
> Furthermore, most of the systems I deal with at work are either 1U or
> blades in datacenter racks with the raised floor forming a plenum to
> deliver temperature-controlled air to each rack, hot and cold isle design, etc.
> I can't observe even a 1C change in temp just by running an individual machine
> or VM at peak load.  Even firing off something which causes a load spike for
> an hour or two across all of the systems or VMs in a particular rack only
> causes a 2-3C change.

And those blades may well have much better internal cooling. 

>
> In practice, that limits thermal wandering due to load from 10-20 PPM to
> around ~2 PPM.
>
>> One of my collegues debvised a script whose sole purpose was to stress
>> the cpus so he could use the air coming out to dry his socks. 
>> Ie, cpu load drives temp change which produces time shifts. 
>
> Yes.  That matters the most for freestanding machines which are not kept
> in air-conditioning.  It matters very little for machines in a data center,
> because the ambient thermals there are controlled fairly precisely.
>
>> And it is there that chrony tends to be better at keeping track of the
>> rate changes.
>
> That's the claim chrony makes, yes.

Actually no, I do not think I have read chronyd making that claim. I
have made that claim for chrony to try to explain the much better stats
that chrony is measured to have. The stats are not a claim, they are
measurements. The explanation is a claim. 
Chrony IS much faster at accounting for fluctuation in the rate or the
offset. That I have also measured. 

>
> Regards,



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