[time-nuts] NTP jitter with Linux

[Dennis Ferguson]

On 4 Apr, 2012, at 16:10 , Mike S wrote:

> On 4/4/2012 6:51 PM, Eric Williams wrote:
>> Could the CPU be reducing its clock rate when it's not being loaded?  Just
>> a guess, most multi-core processors these days have power saving features
>> like that.
>> 
>> On Wed, Apr 4, 2012 at 3:22 PM, Mike S<mikes at flatsurface.com>  wrote:
>>> >  I've played around with different
>>> >  cpufreq setting, thinking it might be related to the processor speed during
>>> >  an IRQ varying, but that seems to have minimal impact (performance vs.
>>> >  conservative vs. ondemand).
> 
> Setting /sys/devices/system/cpu/cpuX/cpufreq/scaling_governor to "performance" should lock that core to the max clock rate.
> 
> In looking that up, I found that the script I made to set this was just doing cpu0 (i.e. one of four cores). Doh! I've changed it to do all 4 cores, and am trying that again to see if that's it.

I don't know much about Linux but if that doesn't help try to find out what
the operating system does in its idle loop.  If it is ending up in some
power-saving state when it is idle it may be volunteering to do this by
executing some magic `wait' instruction which does the power-saving thing as
a side effect, and if you can find where it does this you might be able to
work around it.

Dennis Ferguson