linux-kernel - Re: [PATCH 2/5] mm: remove unlikly NULL from kfree

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Date:	Wed, 25 Mar 2009 16:09:32 -0500
From:	Matt Mackall <mpm@...enic.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Hua Zhong <hzhong@...il.com>, linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Roland McGrath <roland@...hat.com>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Steven Rostedt <srostedt@...hat.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH 2/5] mm: remove unlikly NULL from kfree

On Wed, 2009-03-25 at 13:26 -0700, Jeremy Fitzhardinge wrote:
> Steven Rostedt wrote:
> > Thanks for this info!
> >
> > Although gcc plays a role too. That is, if we have
> >
> > 	if (x)
> > 		do something small;
> >
> > 	do something large;
> >
> >
> > this can be broken into:
> >
> > 	cmp x
> > 	beq 1f
> > 	do something small
> > 1:
> > 	do something large
> >
> > Which plays nice with the cache. But, by adding a unlikely(x), gcc will 
> > probably choose to do:
> >
> > 	cmp x
> > 	bne 2f
> >
> > 1:
> > 	do something large
> >
> > 	ret;
> >
> > 2:
> > 	do something small
> > 	b 1b
> >
> > which hurts in a number of ways.
> >   
> 
> I think that's probably the dominant effect on x86 systems, because 
> Intel doesn't recommend using the branch hint prefixes as far as I can 
> tell (their consumption of icache space outweighs any benefit of priming 
> the predictor).

Yeah, I was actually thinking 'hint' in the gcc sense of adding
unlikely() or not, which results in, say, choosing one code layout vs
the other based on the CPU's cold-cache bias. 

-- 
http://selenic.com : development and support for Mercurial and Linux


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