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

Uoti Urpala uoti.urpala
Tue Dec 16 19:01:34 CET 2008

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.

> if i remove some unneeded code and that results in a 0.5% gain on one machine
> chances are it also does on most others, its not as if the removial of
> code will likely make it slower.

You can expect that removal of useless code won't make things slower _on
average_. However if the CPU use of the removed code was significantly
less than 0.5% of the total then you can also expect the consistent
effect to be masked by random variation - something like 50.1% of
machines showing better benchmark results and 49.9% showing worse.

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