Re: [PATCH RFC] smt nice introduces significant lock contention

[Con Kolivas <kernel@kolivas.org>]

On Saturday 03 June 2006 07:54, Chen, Kenneth W wrote:
> Con Kolivas wrote on Friday, June 02, 2006 6:17 AM
> > Thinking some more on this it is also clear that the concept of
> > per_cpu_gain for smt is basically wrong once we get beyond straight
> > forward 2 thread hyperthreading. If we have more than 2 thread units per
> > physical core, the per cpu gain per logical core will decrease the more
> > threads are running on it. While it's always been obvious the gain per
> > logical core is entirely dependant on the type of workload and wont be a
> > simple 25% increase in cpu power, it is clear that even if we assume an
> > "overall" increase in cpu for each logical core added, there will be some
> > non linear function relating power increase to thread units used. :-|
>
> In the context of having more than 2 sibling CPUs in a domain, doesn't the
> current code also suffer from thunder hurd problem as well? When high
> priority task goes to sleep, it will wake up *all* sibling sleepers and
> then they will all fight for CPU resource, but potentially only one will
> win?

Yes. The smt nice code was never designed with that many threads in mind. This 
is why I'm bringing it up for discussion.

-- 
-ck