Re: [RFC] (How to) Let idle CPUs sleep

[Srivatsa Vaddagiri <vatsa@in.ibm.com>]

On Thu, May 12, 2005 at 09:28:55AM -0700, Jesse Barnes wrote:
> Seems like we could schedule timer interrupts based solely on add_timer 
> type stuff; the scheduler could use it if necessary for load balancing 
> (along with fork/exec based balancing perhaps) on large machines where 
> load imbalances hurt throughput a lot.  But on small systems if all 

Even if we were to go for this tickless design, the fundamental question
remains: who wakes up the (sleeping) idle CPU upon a imbalance? Does some other
(busy) CPU wake it up (which makes the implementation complex) or the idle CPU 
checks imbalance itself at periodic intervals (which restricts the amount of
time a idle CPU may sleep).

> your processes were blocked, you'd just go to sleep indefinitely and 
> save a bit of power and avoid unnecessary overhead.
> 
> I haven't looked at the lastest tickless patches, so I'm not sure if my 
> claims of simplicity are overblown, but especially as multiprocessor 
> systems become more and more common it just seems wasteful to wakeup 
> all the CPUs every so often only to have them find that they have 
> nothing to do.

I guess George's experience in implementing tickless systems is that
it is more of an overhead for a general purpose OS like Linux. George?


-- 


Thanks and Regards,
Srivatsa Vaddagiri,
Linux Technology Center,
IBM Software Labs,
Bangalore, INDIA - 560017