[mad-dev] imdct36 CPU usage increase

[Derk-Jan Hartman]

On 15 feb 2004, at 02:56, Rob Leslie wrote:

> On Feb 14, 2004, at 7:30 AM, Derk-Jan Hartman wrote:
>> For some strange reason our latest version of VLC OSX more than 
>> doubles the CPU usage of mad_frame_decode, specifically the imdct36 
>> routine it would seem. It is an OS X problem only and very annoying. 
>> Decoding mp3 now requires 28% CPU usage on my G4/400 (used to be 
>> 12%).
>>
>> libmad library version changes have been ruled out as the cause of 
>> this. Our mp3 decoder module didn't change either, but the demux 
>> module did change. We suspect it might be something like an alignment 
>> issue, but we can't seem to track it down..
>>
>> Could you give some advice as to what might be the cause of this 
>> sudden increase in CPU usage, and how me may check some of our input 
>> for correctness?
>
> I don't think alignment should be an issue since nothing in imdct36() 
> depends on the alignment of the bitstream.
>
> The only possibility I can see affecting the performance of imdct36() 
> might be a possible change in compilation directives. The preprocessor 
> symbols FPM_*, OPT_SPEED, and OPT_ACCURACY can all drastically affect 
> performance, as can the selection of compiler optimization flags. You 
> might check to make sure these have not changed.

i configured mad to be as fast as possible and still it's twice as 
intensive as in the older release.
I'm working with profiling data here, so it's a bit hard to detect the 
problem. However...
Though imdct36 is taking the most time by far, I noticed that the 
difference is within III_decode()
III_decode takes almost the same CPU time in both versions. however 
then the differences start.

III_imdct_l takes 2.7% in old version and 7.8% in the new version. That 
imdct36() is called more often because of this is logical of course. So 
the problem is probably somewhere within III_decode() and causing 
III_imdct_l to be called more often than before...
Any new ideas after this information?

DJ


---
Universiteit Twente
Derk-Jan Hartman (d.hartman at student.utwente dot nl)
http://home.student.utwente.nl/d.hartman