linux-kernel - Re: -rt dbench scalabiltiy issue

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Date:	Wed, 18 Nov 2009 11:52:46 +0100
From:	Nick Piggin <npiggin@...e.de>
To:	Thomas Gleixner <tglx@...utronix.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

On Wed, Nov 18, 2009 at 11:19:14AM +0100, Thomas Gleixner wrote:
> 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.

OK well then you don't reduce atomic ops in the lookup/dput fastpaths
by protecting d_count with d_lock, so single threaded performance should
not hurt by using atomic_t here.

I'll keep this in mind. As I said, I still need to do some more work on
the fast path lookup and other single threaded performance. In the worst
case that mainline really doesn't like atomic_t there it probably isn't
hard to make some small wrappers for -rt.

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

Yeah I'm sure you're right. I'm just interested where it is coming from
in -rt.

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