linux-kernel - Re: [PATCH [RT] 10/14] adjust pi

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Message-Id: <1203613744.4232.32.camel@hermosa.morrealenet>
Date:	Thu, 21 Feb 2008 10:09:04 -0700
From:	"Peter W. Morreale" <pmorreale@...ell.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Gregory Haskins <ghaskins@...ell.com>, mingo@...e.hu,
	a.p.zijlstra@...llo.nl, tglx@...utronix.de,
	linux-rt-users@...r.kernel.org, linux-kernel@...r.kernel.org,
	bill.huey@...il.com, kevin@...man.org, cminyard@...sta.com,
	dsingleton@...sta.com, dwalker@...sta.com, npiggin@...e.de,
	dsaxena@...xity.net, ak@...e.de, gregkh@...e.de,
	sdietrich@...ell.com, mkohari@...ell.com
Subject: Re: [PATCH [RT] 10/14] adjust pi_lock usage in wakeup

On Thu, 2008-02-21 at 11:48 -0500, Steven Rostedt wrote:
> On Thu, 21 Feb 2008, Gregory Haskins wrote:
> 
> > From: Peter W.Morreale <pmorreale@...ell.com>
> >
> > In wakeup_next_waiter(), we take the pi_lock, and then find out whether
> > we have another waiter to add to the pending owner.  We can reduce
> > contention on the pi_lock for the pending owner if we first obtain the
> > pointer to the next waiter outside of the pi_lock.
> >
> > This patch adds a measureable increase in throughput.
> 
> I see how this may decrease contention (slightly less time in holding the
> pi_lock). But, please, when stating something like: "adds a measurable
> increase in throughput", show the benchmark numbers.
> 
> -- Steve
> 

Approximately 3% to the dbench benchmark we used.

My "standard" sanity check was to mount a ramfs filesystem and execute:

dbench -t 10 30

five times and generate an average from the reported "Throughput"
numbers displayed by the runs. 

dbench was chosen merely because of the contention on dcache_lock.

Best,
-PWM


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