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

["Chen, Kenneth W" <kenneth.w.chen@intel.com>]

Con Kolivas wrote on Friday, June 02, 2006 6:17 AM
> On Friday 02 June 2006 20:30, Con Kolivas wrote:
> > On Friday 02 June 2006 18:56, Nick Piggin wrote:
> > > And why do we lock all siblings in the other case, for that matter? (not
> > > that it makes much difference except on niagara today).
> >
> > If we spinlock (and don't trylock as you're proposing) we'd have to do a
> > double rq lock for each sibling. I guess half the time double_rq_lock will
> > only be locking one runqueue... with 32 runqueues we either try to lock all
> > 32 or lock 1.5 runqueues 32 times... ugh both are ugly.
> 
> 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?