[FFmpeg-devel] [RFC] H.264/SQV3 separation: h264data.h

[Michael Niedermayer]

On Tue, Dec 16, 2008 at 09:09:55PM +0200, Uoti Urpala wrote:
> On Tue, 2008-12-16 at 10:55 -0800, Baptiste Coudurier wrote:
> > Uoti Urpala wrote:
> > > On Tue, 2008-12-16 at 13:31 +0100, Michael Niedermayer wrote:
> > >> On Tue, Dec 16, 2008 at 11:01:34AM +0100, Panagiotis Issaris wrote:
> > >>> Besides that, in my opinion you can't benchmark code on one particular
> > >>> machine and expect a 0.5% performance loss to say anything about the
> > >>> codes performance on other machines (other then that the code hasn't
> > >>> introduced a substantial performance change on similar machines).
> > >> What evidence is there to support this claim?
> > > 
> > > Many known cases where changes that are known not to affect the real
> > > quality of the code alter performance by more than 0.5%. I just tried
> > > adding an unused global function to h264.c. The first attempt didn't
> > > show any quickly measurable difference. Then I made the function a bit
> > > larger, and now the benchmark ran consistently 0.8% faster. Removing the
> > > unused function made things correspondingly 0.8% slower again.
> > 
> > Could we please see the code ?
> 
> I deleted it already - it was just random stuff to add code size. But
> you can get equivalent effects in a few tries. Just add a global (so
> it's not optimized away) function in the middle of h264.c and write
> random stuff there.

What iam wondering is how a speed effect from random unused code on a
single machine is related to your claim above of lack of correlations
between the speed on different machines.

One actual case here, that is the table split showed a slowdown consistant
across machines, thus contradicting your claim.

[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

I know you won't believe me, but the highest form of Human Excellence is
to question oneself and others. -- Socrates
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