On Dec 7, 2014, at 7:19 PM, William Unruh <un...@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. Regards, -- -Chuck _______________________________________________ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions