[time-nuts] x86 CPU Timekeeping and clock generation

[Hal Murray]

> 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.


-- 
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