[Vorbis] Problem with CPU usage on Playstation 2 when using OggVorbis.

[Daniel Collin]

>>> I get CPU usage that looks like this (when i say frames i mean 
>>> actually rendering frames on the
>>> Playstations 2 and you have 50 or 60 frames per second)
>>
>
>>> cpu usage:        ~100%         ~0%         ~0%          0%         
>>> ~100%
>>> current frame:        0                1             2               
>>> 3                4
>>
>
> The fundamental problem is Vorbis can use window sizes larger than the 
> amount of samples than naturally want to be processed each 60 Hz frame 
> draw. E.g. at 48 Khz, about 800 samples would be processed each frame 
> draw. Vorbis doesn't use window sizes of 800. If it did (in a dream 
> world), you wouldn't be having issues.
>
> I believe Vorbis will often use window sizes of 2048. So your peak CPU 
> hit in a frame draw will be 2048/800 percent above the average CPU load.
>
> One thing you could try doing is modifying the encoder to only use 
> frame sizes as large as 1024. This will cut your peak CPU in half. You 
> could go to 512, but I don't think it would help the worst peaks as 
> you will have to decode 2 512 size windows sometimes. Even 256 
> wouldn't help the peak as you would still need 4 of them. 128 would 
> start to see some returns as you would need at most 7. But at that 
> point you are reducing the efficiency of the compression considerably, 
> and you probably eat any CPU savings back processing those small 
> windows. The smaller numbers below 512 only get you a more even CPU 
> distribution between frames (which doesn't do much good in video games 
> as it's the peak that kills you). Keep in mind limiting to 1024 means 
> you can only play your own Vorbis files, if that's an issue. You also 
> reduce the amount of compression possible.
>
> Other than that, looking at the numbers I think you want to do a pile 
> of optimization as it looks like you are using at least 25% CPU 
> anyway. Unless your PS2 application can afford that CPU (that would be 
> surprising), load balancing improvements are a bit pointless unless 
> you optimize as well. I'm pretty sure libvorbis would run dog slow on 
> a PS2 out of the box.
>
> The idea of running Vorbis on a separate thread at a lower priority is 
> also interesting, although perhaps a bit messy. The biggest issue on 
> PS2 is it is cooperative, so you would have to implement a watchdog 
> interrupt of some sort (e.g. the VBL) that could force the low 
> priority Voribis thread(s) to yield when the CPU is needed again for 
> other threads.
>
> I've always thought it would be pretty easy to produce a version of 
> Vorbis that is more friendly to video games. It would encompass at 
> least a few changes, including the window sizes. Out of the box, 
> Vorbis is not the most friendly format for in game music/SFX in video 
> games (e.g. MP3 slides into video games easier, at least on a 
> technical level). Not trying to push MP3, I'm just saying I think it 
> would be very easy to remove the obstacles in Vorbis to the point 
> where it would behave similar to MP3 in regards to CPU, memory, and 
> usage.
>
> Thanks,
> Dave.
>
>

Hi Dave,

Thanks for a very good and detailed reply. I will certanly try to reduce 
the window size, and yeah a low priority thread may be an idea.
The thing is that 25% CPU usage is actually ok for my purpose as im 
actually not doing a game but a "demo". A demo is sorta of like
a realtime musicvideo so it will mostly be graphics that takes most time 
and that is handled by the other co-CPUs. And yeah, most
games actually cant afford this load, and plays music by streaming 
audiotracks or using adpcm (as the hardware decodes that) which
makes the cpu usage way lower. But i will check it out and see what i 
can come up with.

Thanks again.
Daniel