[time-nuts] x86 CPU Timekeeping and clock generation

[Marek Dorsic]

NTP has a drift file (usually /var/lib/ntp/ntp.drift or /var/lib/ntp/drift on Linux) where it stores and periodically updates the computers clock drift measured by ntpd in ppm.
In your scenario I assume it should contain only one number - 0. If it’s not 0, does it correspond to the drift you are observing?
 
   .md

> On 6 Jan 2021, at 08:28, Hal Murray <hmurray at megapathdsl.net> wrote:
> 
> 
>> My question is: what i'm missing? 
> 
> Two ideas come to mind.
> 
> Most PCs (and servers) smear the CPU clock frequency slightly to dance around 
> the FCC rules.  The chip that does that will have slight temperature influence 
> so even if everything else is working right there will be tiny changes if you 
> look closely enough.
> 
> On Linux, you will see something like this at boot time.  (look in dmesg)
> [    0.000000] tsc: Detected 3292.448 MHz processor
> 
> Even if the hardware does the right thing, the software may screwup.  If your 
> system is using the TSC for timekeeping, that number above is used to setup 
> the conversion from TSC ticks to ns.  A year or 6 ago, there was a bug in that 
> routine.  If you patched the boot code to call that routine a half dozen times 
> you would get a half dozen different answers.  The kernel guys didn't notice 
> because they were all close enough that ntpd could compensate.  But any geek 
> looking at ntpd graphs of drift would notice big jumps when the system was 
> rebooted.
> 
> 
> -- 
> These are my opinions.  I hate spam.
> 
> 
> 
> 
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