Re: -rt dbench scalabiltiy issue

[Thomas Gleixner]

Nick,

On Wed, 18 Nov 2009, Nick Piggin wrote:
> > So yes, on -rt, the overhead from lock contention is way way worse then
> > any extra atomic ops. :)
> 
> How about overhead for an uncontended lock? Ie. is the problem caused
> because lock *contention* issues are magnified on -rt, or is it
> because uncontended lock overheads are higher? Detailed callgraph
> profiles and lockstat of +/-atomic case would be very interesting.

In the uncontended case we have the overhead of calling might_sleep()
before we acquire the lock with cmpxchg(). The uncontended unlock is a
cmpxchg() as well.

I don't think that this is significant overhead and we see real lock
contention issues magnified by at least an order of magnitude.

Thanks,

	tglx
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