On 2014-12-08, Charles Swiger <cswi...@mac.com> wrote: > 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".
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, _______________________________________________ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
