Re: No 100 HZ timer !

Oliver Xymoron
2001-08-02T18:44:12Z

On Thu, 2 Aug 2001, george anzinger wrote:

> Oliver Xymoron wrote:
> >
> > Does the higher timer granularity cause overall throughput to improve, by
> > any chance?
> >
> Good question. I have not run any tests for this. You might want to do
> so. To do these tests you would want to build the system with the tick
> less timers only and with the instrumentation turned off. I would like
> to hear the results.
>
> In the mean time, here is a best guess. First, due to hardware
> limitations, the longest time you can program the timer for is ~50ms.
> This means you are reducing the load by a factor of 5. Now the load
> (i.e. timer overhead) is ~0.12%, so it would go to ~0.025%. This means
> that you should have about 0.1% more available for thru put. Even if we
> take 10 times this to cover the cache disruptions that no longer occur,
> I would guess a thru put improvement of no more than 1%. Still,
> measurements are better that guesses...

That's not what I'm getting at at all. Simply raising HZ is known to
improve throughput on many workloads, even with more reschedules: the
system is able to more finely adapt to changes in available disk and
memory bandwidth.

BTW, there are some arguments that tickless is worth doing even on old
PIC-only systems:

http://groups.google.com/groups?q=oliver+xymoron+timer&hl=en&group=mlist.linux.kernel&safe=off&rnum=2&selm=linux.kernel.Pine.LNX.4.30.0104111337170.32245-100000%40waste.org

And I found this while I was looking too:

http://groups.google.com/groups?q=oliver+xymoron+timer&hl=en&group=mlist.linux.kernel&safe=off&rnum=3&selm=linux.kernel.Pine.LNX.4.10.10010241534110.2957-100000%40waste.org

..but no one thought it was interesting at the time.

--
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