Re: One of these things (CONFIG_HZ) is not like the others..

[John Stultz]

On 01/27/2013 10:08 PM, Santosh Shilimkar wrote:
> On Tuesday 22 January 2013 08:35 PM, Santosh Shilimkar wrote:
> > On Tuesday 22 January 2013 08:21 PM, Russell King - ARM Linux wrote:
> > > On Tue, Jan 22, 2013 at 03:44:03PM +0530, Santosh Shilimkar wrote:
> > > > Sorry for not being clear enough. On OMAP, 32KHz is the only clock 
> > > > which
> > > > is always running(even during low power states) and hence the clock
> > > > source and clock event have been clocked using 32KHz clock. As 
> > > > mentioned
> > > > by RMK, with 32768 Hz clock and HZ = 100, there will be always an
> > > > error of 0.1 %. This accuracy also impacts the timer tick interval.
> > > > This was the reason, OMAP has been using the HZ = 128.
> > >
> > > Ok.  Let's look at this.  As far as time-of-day is concerned, this
> > > shouldn't really matter with the clocksource/clockevent based system
> > > that we now have (where *important point* platforms have been converted
> > > over.)
> > >
> > > Any platform providing a clocksource will override the jiffy-based
> > > clocksource.  The measurement of time-of-day passing is now based on
> > > the difference in values read from the clocksource, not from the actual
> > > tick rate.
> > >
> > > Anything _not_ providing a clock source will be reliant on jiffies
> > > incrementing, which in turn _requires_ one timer interrupt per jiffies
> > > at a known rate (which is HZ).
> > >
> > > Now, that's the time of day, what about jiffies?  Well, jiffies is
> > > incremented based on a certain number of nsec having passed since the
> > > last jiffy update.  That means the code copes with dropped ticks and
> > > the like.
> > >
> > > However, if your actual interrupt rate is close to the desired HZ, then
> > > it can lead to some interesting effects (and noise):
> > >
> > > - if the interrupt rate is slightly faster than HZ, then you can end up
> > >    with updates being delayed by 2x interrupt rate.
> > > - if the interrupt rate is slightly slower than HZ, you can 
> > > occasionally
> > >    end up with jiffies incrementing by two.
> > > - if your interrupt rate is dead on HZ, then other system noise can 
> > > come
> > >    into effect and you may get maybe zero, one or two jiffy increments
> > > per
> > >    interrupt.
> > >
> > > (You have to think about time passing in NS, where jiffy updates should
> > > be vs where the timer interrupts happen.)  See 
> > > tick_do_update_jiffies64()
> > > for the details.
> > >
> > > The timer infrastructure is jiffy based - which includes scheduling 
> > > where
> > > the scheduler does not use hrtimers.  That means a slight discrepency
> > > between HZ and the actual interrupt rate can cause around 1/HZ jitter.
> > > That's a matter of fact due to how the code works.
> > >
> > > So, actually, I think the accuracy of HZ has much overall effect
> > > _provided_
> > > a platform provides a clocksource to the accuracy of jiffy based timers
> > > nor timekeeping.  For those which don't, the accuracy of the timer
> > > interrupt to HZ is very important.
> > >
> > > (This is just based on reading some code and not on practical
> > > experiments - I'd suggest some research of this is done, trying HZ=100
> > > on OMAP's 32kHz timers, checking whether there's any drift, checking
> > > how accurately a single task can be woken from various 
> > > select/poll/epoll
> > > delays, and checking whether NTP works.)
> > Thanks for expanding it. It is really helpful.
> >
> > > And I think further discussion is pointless until such research has 
> > > been
> > > done (or someone who _really_ knows the time keeping/timer/sched code
> > > inside out comments.)
> > Fully agree about experimentation to re-asses the drift.
> >  From what I recollect from past, few OMAP customers did
> > report the time drift issue and that is how the switch
> > from 100 --> 128 happened.
> >
> > Anyway I have added the suggested task to my long todo list.
> So I tried to see if any time drift with HZ = 100 on OMAP. I ran the
> setup for 62 hours and 27 mins with time synced up once with NTP server.
> I measure about ~174 millisecond drift which is almost noise considering
> the observed duration was ~224820000 milliseconds.

So 174ms drift doesn't sound great, as < 2ms (often much less - though 
that depends on how close the server is) can be expected with NTP. 
Although its not clear how you were measuring: Did you see a max 174ms 
offset while trying to sync with NTP? Was that offset shortly after 
starting NTP or after NTP converged down?

thanks
-john

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