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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
Date: Wed, 18 Nov 2009 11:19:14 +0100 (CET)	[thread overview]
Message-ID: <alpine.LFD.2.00.0911181110430.24119@localhost.localdomain> (raw)
In-Reply-To: <20091118042516.GC21813@wotan.suse.de>

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

  reply	other threads:[~2009-11-18 10:19 UTC|newest]

Thread overview: 17+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2009-10-16 20:05 john stultz
2009-10-17  0:45 ` Paul E. McKenney
2009-10-17  1:03   ` john stultz
2009-10-17  1:37     ` john stultz
2009-10-17 23:06       ` Nick Piggin
2009-10-17 22:39 ` Nick Piggin
2009-11-18  1:28   ` john stultz
2009-11-18  4:25     ` Nick Piggin
2009-11-18 10:19       ` Thomas Gleixner [this message]
2009-11-18 10:52         ` Nick Piggin
2009-11-20  2:22       ` john stultz
2009-11-23  9:06         ` Nick Piggin
2009-11-25  2:16           ` john stultz
2009-11-25  7:18             ` Nick Piggin
2009-11-25 22:20               ` john stultz
2009-11-26  6:20                 ` Nick Piggin
2009-12-02  1:53                   ` john stultz

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