Re: CPU affinity & IPI latency

[Davide Libenzi]

On 13-Jul-2001 Mike Kravetz wrote:
> On Fri, Jul 13, 2001 at 09:41:44AM -0700, Davide Libenzi wrote:
>> 
>> I personally think that a standard scheduler/cpu is the way to go for SMP.
>> I saw the one IBM guys did and I think that the wrong catch there is trying
>> always to grab the best task to run over all CPUs.
> 
> That was me/us.  Most of the reason for making 'global scheduling'
> decisions was an attempt to maintain the same behavior as the existing
> scheduler.  We are trying to see how well we can make this scheduler
> scale, while maintaining this global behavior.  Thought is that if
> there was ever any hope of someone adopting this scheduler, they would
> be more likely to do so if it attempted to maintain existing behavior.

The problem, IMHO, is that we're trying to extend what is a correct behaviour on
the UP scheduler ( pickup the best task to run ) to SMP machines.
Global scheduling decisions should be triggered in response of load unbalancing
and not at each schedule() run otherwise we're going to introduce a common lock
that will limit the overall scalability.
My idea about the future of the scheduler is to have a config options users can
chose depending upon the machine use.
By trying to keep a unique scheduler for both UP and MP is like going to give
the same answer to different problems and the scaling factor ( of the scheduler
itself ) on SMP will never improve.
The code inside kernel/sched.c should be reorganized ( it contains even not
scheduler code ) so that the various CONFIG_SCHED* will not introduce any messy
inside the code ( possibly by having the code in separate files
kernel/sched*.c ).

- Davide