On Feb 12, 2012, at 9:36 AM, unruh wrote: > On 2012-02-12, Ron Frazier (NTP) <timekeepingntpl...@c3energy.com> wrote: >> It is my understanding that NTP is continuously making small changes to >> the software clock to keep the timing accurate while the os is running. >> 95% of the time, my computers are doing the same thing and 95% of the >> time, I'm doing the same thing with the computers. Therefore, over a >> long time interval, the interrupt usage should be similar, and over a >> long time interval, the correct clock frequency to maintain accuracy >> should be similar. > > That above paragraph is not comprehensible to me. Yes, ntp is making > small changes to the software clock frequency. > What does your doing with the computer have to do with interrupt usage?
I also found it a bit difficult to understand the concern being asked, but: - some operating systems have or had bugs where they will miss timer interrupts and cause the kernel "clock" to run more slowly (ie, a firewall/router running at high packets-per-second and seeing a huge # of network interrupts) - doing some long, max-CPU activity like "transcoding a movie" will heat up the system and the crystal > The clock crystal ages, and suffers internal crystal "cracks" > migrations, etc, which change the frequency of the crystal. Thus even in > a temperature controlled oven, the crystal frequency will change, but > much of the crystal frequency change is driven by temperature changes. Agreed, temperature swings will have a major impact on the crystal frequency. >> I also would like to understand how ntp interacts with the Real Time >> Clock. I think I've read that either NTP or the OS (I don't know which) > > It depends. ntp itself does not intereact with the real time clock at > all. However, under Linux, if the system clock thinks it is synced, it > resets te real time clock every 11 min to the system clock. also, in the > OS, hwclock is run at the end to reset the real time clock to the system > clock. Yes, ntpd does not interact with the hardware TOY/RTC at all. Whether the system itself updates the BIOS/firmware/EFI RTC is both operating system specific and hardware specific. >> will save the time to the RTC when shutting down and retrieve the time >> from the RTC when booting up. I'd like to know if this is true, first >> of all, and I'd like to know if it makes any corrections to the clock >> rate of the RTC so it is more accurate. > > No. it does not, especially with that 11 minute mode, it cannot figure > out the rate of the clock. If you switch off the 11 min mode, by > constantly telling the system clock it is not in sync, then you can use > some versions of hwclock to measure the drift rate of the rtc. > But there is absolutely no way of altering the rate of the rtc without > unsoldering your clock crystal from the motherboard and putting in a new > one, or putting in a trimming capacitor, and adjusting it by hand. You might be able to improve the stability of the crystal by ensuring good airflow and cooling via HVAC as needed. And I suppose you could adjust the rate by changing the HVAC set-point, but I don't think the benefit is worth it. I'd be more likely switch to an OXCO + GPS on a PCI/PCIe card if such as needed. Regards, -- -Chuck _______________________________________________ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
