Date: Wed, 18 Nov 2009 11:19:14 +0100 (CET)
From: Thomas Gleixner <tglx@linutronix.de>
To: Nick Piggin <npiggin@suse.de>
cc: john stultz <johnstul@us.ibm.com>, Ingo Molnar <mingo@elte.hu>,
       Darren Hart <dvhltc@us.ibm.com>, Clark Williams <williams@redhat.com>,
       "Paul E. McKenney" <paulmck@us.ibm.com>,
       Dinakar Guniguntala <dino@in.ibm.com>,
       lkml <linux-kernel@vger.kernel.org>
Subject: Re: -rt dbench scalabiltiy issue
In-Reply-To: <20091118042516.GC21813@wotan.suse.de>
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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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