Re: [patch 0/3] j_state_lock, j_list_lock, remove-bitlocks

[Ingo Molnar]

* Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:

> > > ooh, I'd rather not.  I spent an intense three days removing all the
> > > sleeping locks from ext3 (and three months debugging the result).
> > > Ended up gaining 1000% on 16-way.
> > >
> > > Putting them back in will really hurt the SMP performance.
> >
> > ah. Yeah. Sniff.
> >
> > if we gain 1000% on a 16-way then there's something really wrong about
> > semaphores (or scheduling) though. A semaphore is almost a spinlock, in
> > the uncontended case - and even under contention we really (should) just
> > spend the cycles that we'd spend spinning. There will be some
> > intermediate contention level where semaphores hurt, but 1000% sounds
> > truly excessive.
> >
> 
> Could it possibly be that in the process of removing all the sleeping
> locks from ext3, that Andrew also removed a flaw in ext3 itself that
> is responsible for the 1000% improvement?

i think the chances for that are really remote. I think it must have
been a workload ending up scheduling itself to death, while spinlocks
force atomicity of execution and affinity.

we should be able to see the same scenario with PREEMPT_RT on a 16-way
:-)

	Ingo
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