Re: Dynamic ticks make system jerking

[Uwe Kleine-Koenig <ukleinek@informatik.uni-freiburg.de>]

Hello,

Thomas Gleixner wrote:
> On Tue, 2007-06-26 at 17:00 +0200, Uwe Kleine-K??nig wrote:
> > If I enable NO_HZ, the system jerks (I hope this is an understandable
> > term ...).   E.g. 
> > 
> > 	# time find /sys
> > 	...
> > 	real    1m 19.52s
> > 	user    0m 0.18s
> > 	sys     0m 0.15s
> > 
> > on a freshly booted machine.  The output comes in several hunks with
> > pausing between them.  With no_hz=off, I get approx. the same.
> 
> it's nohz=off not no_hz=off
Currently I'm not sure, which I was really using.  I think it was the
right one, because dmesg changed.  But anyhow, I will retest if I made
it wrong.
 
> > First I wondered why set_next_event is called twice between two timer
> > interrupts most of the time.  I found out that the timer is programed
> > for the next tick in any case and if nothing needs the next tick, the
> > interval is enlarged.  I didn't spend time yet to check if it is
> > easier/faster to only program the timer once.
> 
> We optimize for the non-idle path, i.e. we keep the timer running as
> long as we are not idle. Once we go idle we reprogram it.
OK, sounds sane.
 
> I looked at the patch and as far as I can understand it, there is
> nothing obviously wrong about it.
What a pity.

> One remark: why did you expand the clocksource to be 64 bit? The generic
> code handles the 32 bit wrap already, so the expansion in your read
> routine is adding overhead. The counter will wrap every 24 seconds, so
> there is nothing to worry about.
OK, I thought it to be better to have 64bit + some overhead.  I will
change that.
 
> The behavior you are describing is might be related to some problem with
> the clocksource readout, as the reprogramming values are calculated from
> there. 
Any hints how I can debug that?

Currently I cannot retest, but I will dig into it again and rereport.

Thanks for your answer.

Best regards
Uwe

-- 
Uwe Kleine-Koenig

http://www.google.com/search?q=speed+of+light%3D