correct CONFIG_HZ option

[andi.platschek@gmail.com (Andreas Platschek)]

On 06/28/2012 01:21 PM, solmac john wrote:
> Thanks for reply
> what is CONFIG_NO_HZ
Hi!

If you have no idea what a config entry means, use the help in 
menuconfig (or whatever you are using)... e.g. for CONFIG_NO_HZ tells you

----------<snip>--------------
CONFIG_NO_HZ:

  This option enables a tickless system: timer interrupts will
  only trigger on an as-needed basis both when the system is
  busy and when the system is idle.

Symbol: NO_HZ [=n]
Type  : boolean
Prompt: Tickless System (Dynamic Ticks)
    Defined at kernel/time/Kconfig:7
    Depends on: !ARCH_USES_GETTIMEOFFSET && GENERIC_CLOCKEVENTS [=y]
    Location:
      -> Processor type and features
    Selects: TICK_ONESHOT [=n]
----------<snap>-------------

So this already gives you a general idea what the config entry does, and 
some other useful info like dependency on other stuff that has to be 
turn on/off.
> can we enable CONFIG_NO_HZ and CONFIG_HZ both in our config
This question is also answered here, as "Depends on:" would include 
!CONFIG_HZ_250 if it had to be switched or sth.

> and what the drawback of dynamic CONFIG_NO_HZ tick.

I think the problems of longer latencies only apply on idle systems. Not 
100% sure, but I think when the cpu goes into idle, the periodic timer 
is stopped and longer intervals are done using one-shot timer to avoid 
forcing the cpu out of idle just to see that nothing is to do and go 
back into idle.
But I think if the CPU is not in idle the periodic timer.

regards,
andi
>
>
> On Tue, Jun 26, 2012 at 10:47 AM, Mulyadi Santosa 
> <mulyadi.santosa at gmail.com <mailto:mulyadi.santosa@gmail.com>> wrote:
>
>     Hi... :)
>
>     On Tue, Jun 26, 2012 at 12:10 PM, solmac john
>     <johnsolmac at gmail.com <mailto:johnsolmac@gmail.com>> wrote:
>     > I am using ARM multicore board and by default
>
>     okay, I am answering it from what I know about HZ impact on x86...
>
>     > CONFIG_HZ=250
>
>     looks good... a middle safe number, not too high not too low...
>
>     > Query: - 1- How to decide HZ for particular hardware
>
>     it's you who decide...do you want finer grained timer? or coarse one?
>
>     the impact is usually toward latency and responsiveness....together
>     with preemption model you choose actually.
>
>     >               2- Which is the best open source tool to test system
>     > performance from given HZ.
>
>     run your application in that platform and see if it gives you impact.
>     from my experience, unless you need application that is sensitive in
>     timing such as MIDI sequencer, you won't notice the difference.
>
>     --
>     regards,
>
>     Mulyadi Santosa
>     Freelance Linux trainer and consultant
>
>     blog: the-hydra.blogspot.com <http://the-hydra.blogspot.com/>
>     training: mulyaditraining.blogspot.com
>     <http://mulyaditraining.blogspot.com/>
>
>
>
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