Re: TSC vs. HPET timecounter priority

[Erik Fair <fair%netbsd.org@localhost>]

On Aug 13, 2012, at 02:10, Dennis Ferguson wrote:

> 
> On 13 Aug, 2012, at 07:34 , YAMAMOTO Takashi wrote:
>>> What about the following patch?
>>> tc_quality for hpet is set to 2000 so it would have a higher priority.
>>> Using 1500 leave some room for other implementations to have a higher
>>> priority then tsc.
>> 
>> please take a look at tsc_tc_init.
> 
> Yes, it would be better to try to fix that to find more TSC-broken
> cases, including the one's we're talking about, if possible.  If the
> TSC works on your system it is a really good idea to use it.  It is more
> precise, it is much cheaper to sample (the HPET is off-chip) and it can
> potentially provide system call free time to applications.  Making the
> HPET the default in all cases, including those where the TSC works well,
> throws those advantages away for most people.

In your experience, does the spread spectrum or other fuzzing of CPU clock 
frequencies to meet the FCC EMI emissions limit standard degrade the frequency 
stability of TSC (quite aside from the CPU clock frequency changes performed by 
power management functions) for precision timekeeping (e.g. NTP)?

http://en.wikipedia.org/wiki/Phase-locked_loop#Spread_spectrum

http://www.alsc.com/pdf/emi_ssc.pdf

        curious,

        Erik <fair%netbsd.org@localhost>