Re: Periodic clock tick considered harmful (was: Re: HZ, preferably as small as possible)

[Arnd Bergmann]

Thunder from the hill wrote:
> On Thu, 11 Jul 2002 [e-mail] wrote:
>> OK, so I'm just an ignorant member of the peanut gallery, but
>> I'd like to hear a real kernel hacker explain why this isn't
>> the way to go.
> 
> The only thing that was mentioned yet was the amount of stuff that depends
> on periodic ticks. If we just tick unperiodically, we'd fail for sure, but
> if we make these instances depend on another timer - we won.
> 
> I think a good scheduler can handle this and should also be able to
> determine a halfaway optimal tick rate for the current load.

The current approach on s390 is stop the timer tick only for idle cpus,
because that's where it hurts. A busy system can just keep on using 100
(or 1000) Hz timers.
The jiffies value then gets updated from the time stamp counter when an 
interrupt happens on an idle CPU.

See Martin Schwidefsky's recent post for code:

http://marc.theaimsgroup.com/?l=linux-kernel&m=102578746520177&w=2
http://marc.theaimsgroup.com/?l=linux-kernel&m=102578746420174&w=2

        Arnd <><