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

[Jason Baron <jbaron@redhat.com>]

On Thu, Jun 10, 2010 at 12:13:39PM -0400, Mathieu Desnoyers wrote:
> * Jason Baron (jbaron@redhat.com) wrote:
> > On Thu, Jun 10, 2010 at 02:14:40PM +0200, Ingo Molnar wrote:
> > > * Peter Zijlstra <peterz@infradead.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