linux-kernel - Re: -rt dbench scalabiltiy issue

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Message-ID: <alpine.LFD.2.00.0911181110430.24119@localhost.localdomain>
Date:	Wed, 18 Nov 2009 11:19:14 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Nick Piggin <npiggin@...e.de>
cc:	john stultz <johnstul@...ibm.com>, Ingo Molnar <mingo@...e.hu>,
	Darren Hart <dvhltc@...ibm.com>,
	Clark Williams <williams@...hat.com>,
	"Paul E. McKenney" <paulmck@...ibm.com>,
	Dinakar Guniguntala <dino@...ibm.com>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: -rt dbench scalabiltiy issue

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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