linux-kernel - Re: [PATCH 03/13] jump label v9: x86 support

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Message-ID: <20100611005206.GA2379@redhat.com>
Date:	Thu, 10 Jun 2010 20:52:07 -0400
From:	Jason Baron <jbaron@...hat.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	Ingo Molnar <mingo@...e.hu>, Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, hpa@...or.com, tglx@...utronix.de,
	rostedt@...dmis.org, andi@...stfloor.org, roland@...hat.com,
	rth@...hat.com, mhiramat@...hat.com, fweisbec@...il.com,
	avi@...hat.com, davem@...emloft.net, vgoyal@...hat.com,
	sam@...nborg.org
Subject: Re: [PATCH 03/13] jump label v9: x86 support

On Thu, Jun 10, 2010 at 12:13:39PM -0400, Mathieu Desnoyers wrote:
> * Jason Baron (jbaron@...hat.com) wrote:
> > On Thu, Jun 10, 2010 at 02:14:40PM +0200, Ingo Molnar wrote:
> > > * Peter Zijlstra <peterz@...radead.org> wrote:
> > > 
> > > > On Wed, 2010-06-09 at 17:39 -0400, Jason Baron wrote:
> > > > > +       select HAVE_ARCH_JUMP_LABEL if !CC_OPTIMIZE_FOR_SIZE
> > > > 
> > > > That deserves a comment somewhere, it basically makes OPTIMIZE_FOR_SIZE 
> > > > useless...
> > > 
> > > Hm, we need more than a comment for that - distros enable CC_OPTIMIZE_FOR_SIZE 
> > > all the time, for the massive kernel image (and hotpath cache footprint) 
> > > savings. Is this fixable?
> > > 
> > > Thanks,
> > > 
> > > 	Ingo
> > > 
> > 
> > When I tested 'jump label' with CC_OPTIMIZE_FOR_SIZE, I saw a small
> > performance drop , b/c there is less block re-ordering happening.
> 
> Is this a performance drop compared to a jump-label-less kernel or
> compared to -O2 kernel compiled with jump labels ? Or both ?
> 
> Mathieu
> 

Hi Mathieu,

So I'm quoting tbench benchmark here. The performance drop was jump
label vs. all jump label patches backed out on -Os. If we move to -02,
both the no jump label patches and the jump label patches applied are
faster than all jump label patches backed out on -Os.

so:

jump labels -02 > no jump labels -02 > no jump labels -0s > jump lables
-Os

thanks,

-Jason
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