[ntp:questions] Red Hat vote for chrony

Charles Swiger cswiger at mac.com
Mon Dec 8 15:57:30 UTC 2014

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".

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.

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.


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